Methods and systems for power optimization in a medical device

ABSTRACT

A physiological monitoring system may use photonic signals to determine physiological parameters. The system may vary parameters of a light drive signal used to generate the photonic signal from a light source such that power consumption is reduced or optimized. Parameters may include light intensity, firing rate, duty cycle, other suitable parameters, or any combination thereof. In some embodiments, the system may use information from a first light source to generate a light drive signal for a second light source. In some embodiments, the system may vary parameters in a way substantially synchronous with physiological pulses, for example, cardiac pulses. In some embodiments, the system may vary parameters in response to an external trigger.

The present disclosure relates to power optimization, and moreparticularly relates to conserving and optimizing power in aphotoplethysmography system or other medical device.

SUMMARY

Systems and methods are provided for optimizing power consumption in anoptical physiological monitoring system. The system may vary light drivesignal parameters to reduce power consumption or vary power use. Thesystem may vary parameters in a technique correlated to cardiac pulsecycles. In some embodiments, reducing power consumption may allow forincreased battery life in portable systems or increased portability. Insome embodiments, varying light output during a cardiac cycle may reduceheating effects of the emitters. Parameters that may be varied includelight intensity, firing rate, duty cycle, other suitable parameters, orany combination thereof. The generated signals may be used to determinedphysiological parameters such as blood oxygen saturation, hemoglobin,blood pressure, pulse rate, other suitable parameters, or anycombination thereof.

In some embodiments, the system may use information from a first lightsource to control a second light source. The system may generate a firstlight drive signal for activating a first light source to emit a firstphotonic signal. The first light source and second light source may eachinclude one or more emitters. The system may receive a light signalattenuated by the subject, wherein the light signal comprises acomponent corresponding to the first photonic signal. The system mayanalyze the component of the light signal to determine when to activatea second light source. The system may generate a second light drivesignal, based on the analysis of the first component, for activating thesecond light source to emit one or more second photonic signals. Thesystem may determine one or more physiological parameters based on thelight signals.

In some embodiments, the system may vary a light drive signal in a waysubstantially synchronous with physiological pulses, for example,cardiac pulses. The system may generate a light drive signal foractivating a light source to emit a photonic signal, wherein at leastone parameter of the light drive signal is configured to varysubstantially synchronously with physiological pulses of the subject.The system may receive a light signal attenuated by the subject, whereinthe signal comprises a component corresponding to the emitted photonicsignal. The system may determine physiological parameters based on thesignal. In some embodiments, the system may vary light levels with otherperiodic (or mostly periodic) physiological changes. For example, venousreturn changes with intrathoracic pressure during a respiration cyclecan affect the baseline level of the photoplethysmography waveform. Thesystem may vary the emitter output such that similar signal quality isavailable at the detector over time varying volumes of venous bloodpresent in the path of light.

In some embodiments, the system may vary a light drive signal based on areceived external trigger. The system may receive an external triggerbased on a signal other than a light signal received by thephysiological monitor. The trigger may include a signal received from anECG sensor, an ECG sensor configured to detect an R-wave, a bloodpressure sensor, a respiration rate sensor, any other suitable sensor,or any combination thereof. The system may, in response to the externaltrigger, vary the light intensity, duty cycle, light source firing rate,any other suitable parameter, or any combination thereof.

BRIEF DESCRIPTION OF THE FIGURES

The above and other features of the present disclosure, its nature andvarious advantages will be more apparent upon consideration of thefollowing detailed description, taken in conjunction with theaccompanying drawings in which:

FIG. 1 is a block diagram of an illustrative physiological monitoringsystem in accordance with some embodiments of the present disclosure;

FIG. 2A shows an illustrative plot of a light drive signal in accordancewith some embodiments of the present disclosure;

FIG. 2B shows an illustrative plot of a detector signal that may begenerated by a sensor in accordance with some embodiments of the presentdisclosure;

FIG. 2C shows illustrative timing diagrams of a drive cycle modulationand cardiac cycle modulation in accordance with some embodiments of thepresent disclosure;

FIG. 3 is a perspective view of an embodiment of a physiologicalmonitoring system in accordance with some embodiments of the presentdisclosure;

FIG. 4 is a flow diagram showing illustrative steps for determining aphysiological parameter in accordance with some embodiments of thepresent disclosure;

FIG. 5 shows an illustrative timing diagram of a physiologicalmonitoring system in accordance with some embodiments of the presentdisclosure;

FIG. 6 shows another illustrative timing diagram of a physiologicalmonitoring system in accordance with some embodiments of the presentdisclosure;

FIG. 7 shows another illustrative timing diagram of a physiologicalmonitoring system in accordance with some embodiments of the presentdisclosure;

FIG. 8A shows another illustrative timing diagram of a physiologicalmonitoring system in accordance with some embodiments of the presentdisclosure;

FIG. 8B shows another illustrative timing diagram of a physiologicalmonitoring system in accordance with some embodiments of the presentdisclosure;

FIG. 9 is a flow diagram showing illustrative steps for determining aphysiological parameter in accordance with some embodiments of thepresent disclosure;

FIG. 10 shows another illustrative timing diagram of a physiologicalmonitoring system in accordance with some embodiments of the presentdisclosure;

FIG. 11 shows another illustrative timing diagram of a physiologicalmonitoring system in accordance with some embodiments of the presentdisclosure;

FIG. 12 shows another illustrative timing diagram of a physiologicalmonitoring system in accordance with some embodiments of the presentdisclosure;

FIG. 13 shows another illustrative timing diagram of a physiologicalmonitoring system in accordance with some embodiments of the presentdisclosure;

FIG. 14 shows another illustrative timing diagram of a physiologicalmonitoring system in accordance with some embodiments of the presentdisclosure;

FIG. 15 shows another illustrative timing diagram of a physiologicalmonitoring system in accordance with some embodiments of the presentdisclosure;

FIG. 16 shows another illustrative timing diagram of a physiologicalmonitoring system in accordance with some embodiments of the presentdisclosure;

FIG. 17 is a flow diagram showing illustrative steps for decimating andinterpolating a signal in accordance with some embodiments of thepresent disclosure;

FIG. 18 shows an illustrative timing diagram of a physiologicalmonitoring system including sampling rate variation in accordance withsome embodiments of the present disclosure;

FIG. 19 is a flow chart showing steps to adjust a cardiac cyclemodulation based on a physiological condition in accordance with someembodiments of the present disclosure;

FIG. 20 is an illustrative timing diagram of a system operating in afirst and second mode following detection of a physiological conditionin accordance with some embodiments of the present disclosure;

FIG. 21 is another illustrative timing diagram of a system operating ina first and second mode following detection of a physiological conditionin accordance with some embodiments of the present disclosure;

FIG. 22 is a flow diagram showing illustrative steps for identifyingfeatures in a signal in accordance with some embodiments of the presentdisclosure;

FIG. 23 is an illustrative plot of a waveform showing identification offiducials in accordance with some embodiments of the present disclosure;

FIG. 24 is another illustrative plot of a waveform showingidentification of fiducials in accordance with some embodiments of thepresent disclosure;

FIG. 25 is another illustrative plot of a waveform showingidentification of fiducials in accordance with some embodiments of thepresent disclosure;

FIG. 26 is an illustrative plot of waveforms showing pulseidentification in accordance with some embodiments of the presentdisclosure;

FIG. 27 is an illustrative plot of waveforms showing dicrotic notchidentification in accordance with some embodiments of the presentdisclosure; and

FIG. 28 is an illustrative plot of waveforms showing PPG signals inaccordance with some embodiments of the present disclosure.

DETAILED DESCRIPTION OF THE FIGURES

The present disclosure is directed towards power optimization in amedical device. A physiological monitoring system may monitor one ormore physiological parameters of a patient, typically using one or morephysiological sensors. The system may include, for example, a lightsource and a photosensitive detector. Providing a light drive signal tothe light source may account for a significant portion of the system'stotal power consumption. Thus, it may be desirable to reduce the powerconsumption of the light source, while still enabling high qualityphysiological parameters to be determined. The system may reduce thepower consumption by modulating parameters associated with the lightdrive signal in techniques correlated to the cardiac cycle or othercyclical physiological activity. For example, the system may decreasebrightness during a particular portion of the cardiac cycle. It may alsobe desirable to reduce the power consumption by the light drive signalto reduce heating effects caused by an emitter.

An oximeter is a medical device that may determine the oxygen saturationof an analyzed tissue. One common type of oximeter is a pulse oximeter,which may non-invasively measure the oxygen saturation of a patient'sblood (as opposed to measuring oxygen saturation directly by analyzing ablood sample taken from the patient). Pulse oximeters may be included inpatient monitoring systems that measure and display various blood flowcharacteristics including, but not limited to, the oxygen saturation ofhemoglobin in arterial blood. Such patient monitoring systems may alsomeasure and display additional physiological parameters, such as apatient's pulse rate and blood pressure.

An oximeter may include a light sensor that is placed at a site on apatient, typically a fingertip, toe, forehead or earlobe, or in the caseof a neonate, across a foot. The oximeter may use a light source to passlight through blood perfused tissue and photoelectrically sense theabsorption of the light in the tissue. In addition, locations which arenot typically understood to be optimal for pulse oximetry serve assuitable sensor locations for the blood pressure monitoring processesdescribed herein, including any location on the body that has a strongpulsatile arterial flow. For example, additional suitable sensorlocations include, without limitation, the neck to monitor carotidartery pulsatile flow, the wrist to monitor radial artery pulsatileflow, the inside of a patient's thigh to monitor femoral arterypulsatile flow, the ankle to monitor tibial artery pulsatile flow, andaround or in front of the ear. Suitable sensors for these locations mayinclude sensors for sensing absorbed light based on detecting reflectedlight. In all suitable locations, for example, the oximeter may measurethe intensity of light that is received at the light sensor as afunction of time. The oximeter may also include sensors at multiplelocations. A signal representing light intensity versus time or amathematical manipulation of this signal (e.g., a scaled versionthereof, a logarithm taken thereof, a scaled version of a logarithmtaken thereof, a derivative taken thereof, a difference taken thereof,etc.) may be referred to as the photoplethysmograph (PPG) signal. Inaddition, the term “PPG signal,” as used herein, may also refer to anabsorption signal (i.e., representing the amount of light absorbed bythe tissue), a transmission signal (i.e., representing the amount oflight received from the tissue), any suitable mathematical manipulationthereof, or any combination thereof. The light intensity or the amountof light absorbed may then be used to calculate any of a number ofphysiological parameters, including an amount of a blood constituent(e.g., oxyhemoglobin) being measured as well as a pulse rate and wheneach individual pulse occurs.

In some applications, the photonic signal interacting with the tissue isselected to be of one or more wavelengths that are attenuated by theblood in an amount representative of the blood constituentconcentration. Red and infrared (IR) wavelengths may be used because ithas been observed that highly oxygenated blood will absorb relativelyless red light and more IR light than blood with a lower oxygensaturation. By comparing the intensities of two wavelengths at differentpoints in the pulse cycle, it is possible to estimate the blood oxygensaturation of hemoglobin in arterial blood.

The system may process data to determine physiological parameters usingtechniques well known in the art. For example, the system may determineblood oxygen saturation using two wavelengths of light and aratio-of-ratios calculation. The system also may identify pulses anddetermine pulse amplitude, respiration, blood pressure, other suitableparameters, or any combination thereof, using any suitable calculationtechniques. In some embodiments, the system may use information fromexternal sources (e.g., tabulated data, secondary sensor devices) todetermine physiological parameters.

In some embodiments, it may be desirable to implement techniques tooptimize power consumption in an oximeter or other system. For example,in a battery powered system, reducing the power requirements may allowfor smaller devices, longer life, or both. In some embodiments, poweringthe light source may include a large amount of the power load a devicemay experience. In some embodiments, variation of parameters in thelight drive signal may enable a particular amount of power to be usedmore efficiently. For example, the brightness of a light source may bedecreased during a less important period and increased during a moreimportant period. In some embodiments, parameter variation may reducethe impact of heating effects caused by a light source on a subject.Techniques to vary the amount of time a light source is turned on, tovary the brightness of the light source, other techniques, or anycombination thereof, may be employed to modify power consumption.

In some embodiments, the brightness of one of more light sources may bemodulated in a technique that is related to the cardiac cycle. Thecardiac cycle is the substantially periodic repetition of events thatoccur, for example, during heartbeats. The cardiac cycle may include asystole period and diastole period. The cardiac cycle may includepressure changes in the ventricles, pressure changes in the atria,volume changes in the ventricles, volume changes in the atria, openingand closing of heart valves, heart sounds, and other cyclic events. Insome embodiments, the heart may enter a non-periodic state, for example,in certain types of arrhythmia and fibrillation.

As used herein, “cardiac cycle modulation” will refer to the modulationtechniques generally correlated to the cardiac cycle. It will beunderstood that cardiac cycle modulation may include modulation alignedwith pulses of the heart, pulses of a particular muscle group, othersuitable pulses, any other suitable physiological cyclical function, orany combination thereof. In some embodiments, the system may use acardiac cycle modulation with a period on the order of the cardiac cycleperiod. For example, the cardiac cycle modulation may repeat everycardiac cycle. In some embodiments, the system may use a cardiac cyclemodulation with a period on the order of some multiple of the cardiaccycle period. For example, the cardiac cycle modulation may repeat everythree cardiac cycles. In some embodiments, the cardiac cycle modulationmay relate to both a cardiac cycle and a respiratory cycle. The cardiaccycle and the respiratory cycle may have a time varying phaserelationship. It will be understood that cardiac cycle modulationtechniques, while generally related to the cardiac cycle, may notnecessarily be precisely correlated to the cardiac cycle and may berelated to predetermined parameters, other physiological parameters,other physiological cycles, external triggers (e.g., respiration), userinput, other suitable techniques, or any combination thereof.

As used herein, “drive cycle modulation” (described below) will refer toa relatively higher frequency modulation technique that the system mayuse to generate one or more wavelengths of intensity signals. Cardiaccycle modulation may have a period of, for example, around 1 second,while drive cycle modulation may have a period around, for example, 1.6milliseconds.

In some embodiments, conventional servo algorithms may be used inaddition to any combination of cardiac cycle modulation and drive cyclemodulation. Conventional servo algorithms may adjust the light drivesignals due to, for example, ambient light changes, emitter and detectorspacing changes, sensor positioning, other suitable parameters, or anycombination thereof. Generally, conventional servo algorithms varyparameters at a slower rate than cardiac cycle modulation. For example,a conventional servo algorithm may adjust drive signal brightness due toambient light every several seconds. The system may use conventionalservo algorithms in part to keep received signal levels within the rangeof an analog to digital converter's dynamic range. For example, a signalwith amplitudes that are large may saturate an analog to digitalconvertor. In response to a signal with high amplitudes, the system mayreduce emitter brightness. In a further example, the quality of a lowamplitude signal may be degraded by quantization noise by an analog todigital converter. In response, the system may increase the emitterbrightness.

In some embodiments, a technique to remove ambient and backgroundsignals may be used in addition to or in place of a power saving lightmodulation scheme. In a drive cycle modulation technique, the system maycycle light output at a rate significantly greater than the cardiaccycle. For example, a drive cycle modulation cycle may include thesystem turning on a first light source, followed by a “dark” period,followed by a second light source, followed by a “dark” period. Thesystem may measure the ambient light detected by the detector during the“dark” period and then subtract this ambient contribution from thesignals received during the first and second “on” periods. In someembodiments, drive cycle modulation may be implemented using timedivision multiplexing as described above, code division multiplexing,carrier frequency multiplexing, phase division multiplexing, feedbackcircuitry, DC restoration circuitry, any other suitable technique, orany combination thereof. For example, the system may use frequencydivision multiplexing in a drive cycle modulation technique. The cardiaccycle modulation may represent a lower frequency envelope function onthe higher frequency drive cycle. For example, cardiac cycle modulationmay be an envelope on the order of 1 Hz superimposed on a 1 kHz sinewave drive cycle modulation.

In some embodiments, the system may use various cardiac cycle modulationschemes to adjust the brightness of a light source controlled by thelight drive signal used in determining physiological parameters. Thesystem may modulate the brightness of the light source using a periodicwaveform, for example, a sinusoidal or triangle wave. The period of thewaveform may be substantially related to the cardiac pulse rate, forexample, in a one-to-one relationship, a two-to-one relationship, anyother suitable relationship, or any suitable combination thereof. Thesystem may align the peak of the modulated light drive signal with aparticular point in the cardiac cycle to improve the quality of thedetermined physiological parameter, for example, it may be aligned withthe diastolic period, the systolic period, the dicrotic notch, any othersuitable point, or any combination thereof. In some embodiments, thesystem may modulate the light drive signal with a square wave function,such that it is at a low brightness level during a first part of thecardiac cycle and a high brightness level during a second part of thecardiac cycle. In some embodiments, the low brightness level may includeturning one or more light sources off.

In some embodiments, the cardiac cycle modulation technique may beselected or varied, for example, based on empirical data. The system maydetermine or vary the phase relationship of a cardiac cycle modulationbased on the determined physiological parameter. For example, the systemmay vary the timing of a cardiac cycle modulation technique to determinepulse identification based on a metric related to the determined pulse,such as a standard deviation. In another example, data points may beanalyzed to determine if a cardiac pulse peak is aligned with a cardiaccycle modulation maximum, and the system may make phase relationshipadjustments accordingly. In some embodiments, more complex variationalgorithms may be used depending on the determined physiologicalparameter. Selections and variations of cardiac cycle modulationtechniques may also be based on empirical data, user input, lookuptables, historical information, other suitable information or anycombination thereof.

In some embodiments, the system may combine cardiac cycle modulationtechniques. For example, the system may use a first cardiac cyclemodulation technique dining a first pulse cycle and a second cardiaccycle modulation technique during a second pulse cycle. More complexselections, alterations, overlapping, and convolving of cardiac cyclemodulation techniques may be used depending, in part, on the determinedphysiological parameter or parameters.

In some embodiments, the system may correct for non-linearity of lightsources. For example, the emitted intensity of light from an LED may notvary linearly with the drive current. The system may account fornon-linearity by adjusting drive signals, by adjusting amplification ofreceived signal gain, by adjusting received signal processing, by anyother suitable method, or any combination thereof. For example, thesystem may adjust the drive signal to an LED to improve the linearity.Corrections may be determined using a calibration step, lookup tablesfor known components, empirical data, any other suitable techniques, orany combination thereof. For example, the emission intensity relative toa drive signal may be known for a particular LED. Information may beencoded in a calibration resistor or non-volatile calibration memoryincluded in the sensor or the system. In another example, the system maycalibrate emission output by comparing the intensity of received signalsgenerated in response to a high current drive signal with thosegenerated in response to a low current drive signal. In someembodiments, the operating range of a component (e.g., an LED) may belimited. In some embodiments, a component may operate with a linearrelationship between drive signal and output intensity within a knownrange of drive signals, and in a non-linear relationship outside thatrange of drive signals.

In some embodiments of cardiac cycle modulation, the system may modulatemultiple light sources using a plurality of modulation techniques. Forexample, in a system with two light sources, the system may operate afirst light source at full or regular brightness, while operating one ormore additional light sources in a switched or otherwise modulated mode.In some embodiments, the system may operate a first light sourceaccording to a first cardiac cycle modulation technique and a secondlight source according to a second cardiac modulation technique. Thefirst and second cardiac cycle modulation techniques may be the same,correlated, or unrelated. In some embodiments, the system may use thefirst light source to determine periods of interest in the cardiaccycle. The system may, according to the periods of interest, poweradditional light sources, alter the modulation of the additional lightsources, perform other suitable power optimization techniques, or anycombination thereof. In some embodiments, the system may include a firstlight source (e.g., a light source powered at full or regularbrightness) of a type that is a more efficient light source than the oneor more additional light sources. For example, the first light sourcemay be a high efficiency infrared (IR) LED while the one or moreadditional light sources may be lower efficiency red LEDs or laserdiodes. In some embodiments, the first light source may be selectedbased on efficiency parameters and information from the first lightsource may be used only to control a second light source. For example, ahighly efficient first light source that is not at a wavelength ofinterest for physiological parameter determination may be used tocontrol one or more second light sources at wavelengths of interest. Inthis case, the light from the first light source may be used only forcontrolling the second light source and not for determiningphysiological parameters.

In some embodiments, the system may use the first light source todetermine a pulse rate or identify elements of the cardiac cycle, andthe system may use the pulse rate or identified elements in part tocontrol modulation of the light drive signal. Identified elements mayinclude peaks, valleys, troughs, notches, fiducial points, othersuitable elements, or any combination thereof. Fiducial points may berelated to the zero crossings of first and higher order derivatives ofthe waveform. In some embodiments, the system may modulate the firstlight drive signal according to a first cardiac cycle modulationtechnique and may modulate the one or more additional light drive signalaccording to a second cardiac cycle modulation technique. For example,the system may operate the first light source at full or regularbrightness for a first “on” period, and then “off” for a second period.The system may use the first “on” period to adjust or calibrate a secondmodulation technique. The system may implement the second modulationtechnique for the “off” period, using, for example, one or more lightsources that may or may not include the first light source. As describedabove, this cardiac cycle modulation may be implemented in addition to adrive cycle modulation, conventional servo algorithms, or anycombination thereof.

As used herein, the terms “on” and “off” are merely exemplary and maynot necessarily refer to a fully on or off state. For example, “on” and“off” may refer to switching power or other components, high and lowbrightness output states, high and low values within a continuousmodulation, high and low values of electrical current provided to anemitter, high and low values of a duty cycle, high and low values of adecimation ratio (i.e., how often an emitter is switched on), any othersuitable relatively distinct states, or any combination thereof. In someembodiments, “on” and “off” states may relate to high and low values ofa variable that varies with multiple discrete steps. For example, anemitter brightness may be provided by the system as off, low, medium,and high. In this example, an “off” state may refer to off or lowemission and “on” may refer to medium or high. In another example, “off”may refer to an off state and “on” may refer to a low, medium, or highoutput depending on a second input or system variable.

In some embodiments, historical information may be used to determine thetiming of cardiac cycle modulation. For example, information fromprevious pulse cycles may be used to determine “on” and “off” states. Insome embodiments, the system may use statistical information fromhistorical information, for example, mean period and/or standarddeviation of one or more previous pulse cycles. The system may use amean period to determine or estimate the time period between a previousperiod of interest and the next period of interest. For example, thesystem may wait a particular percentage (e.g., 80%) of the mean periodfollowing a period of interest before returning to an “on” state. Insome embodiments, the particular percentage or other criteria may bebased on statistical information. For example, a smaller standarddeviation in the period of historical pulses may indicate that there isrelatively less variation in the pulse period. The system may increasethe amount of time it waits before turning a drive signal back to an“on” state, as the confidence of the position in time of the next periodof interest is high. Similarly, the system may reduce the waiting periodin response to a relatively high standard deviation in the period ofhistorical pulses. For example, the system may identify a relativelyhigh standard deviation in the period of historical pulses when asignificant respiratory sinus arrhythmia is present. In someembodiments, the system may remain in a particular cardiac cyclemodulation mode for an amount of time following a historical event. Forexample, the system may operate in a high power mode without cardiaccycle modulation for a certain time period following, for example, highnoise levels, a loss of signal, or an irregular cardiac rhythm. In someembodiments, the system may use a cardiac cycle modulation duringperiodic abnormal rhythms such as a 2nd degree AC block, bundle branchblock, or sustained ventricular tachycardia.

The system may use one or more cardiac cycle modulation techniquesdepending on the desired physiological parameter. In some embodiments,the system may emit light at a relatively higher brightness level duringa diastole period when the desired physiological parameter is pulseidentification. In some embodiments, the system may emit light at arelatively higher brightness level during a systole period when thedesired physiological parameter is a quantification of pulse amplitudevariability. In some embodiments, the system may emit light at arelatively higher brightness level during a systole period when thedesired physiological parameter is blood oxygen saturation calculatedusing a ratio-of-ratios calculation. In some embodiments, the system mayrequire sampling an accurate time and amplitude for the peak and foot ofa pulse and less accurate sampling of the rising or falling waveform,and may modulate the emitted light accordingly. In some embodiments, thesystem may emit light at a relatively higher level at a time in thecardiac cycle correlated with dicrotic notches, fiducial points, orother points of interest. Fiducial points may include, for example,local maxima, local minima, points related to the zero crossings offirst and higher order derivatives, other points of interest, or anycombination thereof.

In some embodiments, the system may vary the algorithm used to determinea physiological parameter based, in part, on the cardiac cyclemodulation technique. For example, if a cardiac cycle modulationtechnique only detects the peaks and valleys of a pulse cycle, a firsttype of blood oxygen saturation algorithm may be used (e.g., discreteoximetry based only on the peak and valley information). If the cardiaccycle modulation technique detects the entire pulse, a different bloodoxygen saturation detection algorithm may be used (e.g., a regressionbased algorithm).

In some embodiments, the system may alter the cardiac cycle modulationtechnique based on the level of noise, ambient light, other suitablereasons, or any combination thereof. The system may receive, forexample, an increased level of background noise in the signal due topatient motion. The system may increase the brightness of the lightsources in response to the noise to improve the signal-to-noise ratio.In some embodiments, the system may increase brightness throughout thecardiac cycle because the system may require increased signal amplitudesto differentiate between fiducial and other points of interest relatedto physiological parameters and those related to noise or motion. Insome embodiments, the system may change from a modulated light output toa constant light output in response to noise, patient motion, or ambientlight.

In some embodiments, the system may alter the cardiac cycle modulationtechnique based on a determined physiological condition. For example,the system may detect non-periodic cardiac behavior (e.g., arrhythmia,fibrillation, or asystole) and change the modulation technique from amodulated light output to a constant light output. It will be understoodthat pulseless electrical activity, asystole, and/or other electricalarrhythmias may result in ECG activity but not result in detectablepulsatile activity.

In some embodiments, the system may use external triggering to controlor modify a cardiac cycle modulation technique. For example, the systemmay use information from a second sensor such as an ECG sensor, invasiveblood pressure sensor, a second pulse oximeter, a secondphotoplethysmography sensor, a pulse meter, a respiration sensor, anyother suitable sensor, or any combination thereof. For example, theexternal signal may be received from an external ECG sensor configuredto provide a trigger signal synchronous with an element of the cardiaccycle such as an R wave. In some embodiments, the system may receive anexternal trigger from user input or an external processing device. Insome embodiments, the system may correlate a cardiac cycle modulationwith one or more particular points in an ECG signal. In someembodiments, the system may use an algorithm to determine the delaybetween the external signal and points of interest. For example, thesystem may use an algorithm to determine a delay between an ECG R-waveand the fiducial points of interest in a photoplethysmography signalsuch as the peak of the PPG waveform.

In some embodiments, the system may optimize power consumption byvarying a sampling rate. The system may digitize a received signal usingan analog to digital converter operating at a particular rate. In someembodiments, the digitizer rate may be constant. In some embodiments,the digitizer rate may be modulated using a technique correlated to acardiac cycle modulation. For example, the system may sample at a highrate during a period of interest and at a low rate during other periods.In some embodiments, the system may modulate both a light drive signaland a sampling rate. The modulations of the light drive signal and thesampling rate may be correlated. For example, the system may sample thereceived signal at a low rate during a period of low light output and ata high rate during a period of high light output. The system maydecimate or interpolate the digitized signal such that the rate of theprocessed signal is constant.

The following description and accompanying FIGS. 1-28 provide additionaldetails and features of some embodiments of power optimization in amedical device.

FIG. 1 is a block diagram of an illustrative physiological monitoringsystem 100 in accordance with some embodiments of the presentdisclosure. System 100 may include a sensor 102 and a monitor 104 forgenerating and processing physiological signals of a subject. In someembodiments, sensor 102 and monitor 104 may be part of an oximeter.

Sensor 102 of physiological monitoring system 100 may include lightsource 130 and detector 140. Light source 130 may be configured to emitphotonic signals having one or more wavelengths of light (e.g., Red andIR) into a subject's tissue. For example, light source 130 may include aRed light emitting light source and an IR light emitting light source,e.g., Red and IR light emitting diodes (LEDs), for emitting light intothe tissue of a subject to generate physiological signals. In oneembodiment, the Red wavelength may be between about 600 mm and about 700nm, and the IR wavelength may be between about 800 nm and about 1000 nm.It will be understood that light source 130 may include any number oflight sources with any suitable characteristics. In embodiments where anarray of sensors is used in place of single sensor 102, each sensor maybe configured to emit a single wavelength. For example, a first sensormay emit only a Red light while a second may emit only an IR light.

It will be understood that, as used herein, the term “light” may referto energy produced by radiative sources and may include one or more ofultrasound, radio, microwave, millimeter wave, infrared, visible,ultraviolet, gamma ray or X-ray electromagnetic radiation. As usedherein, light may also include any wavelength within the radio,microwave, infrared, visible, ultraviolet, or X-ray spectra, and thatany suitable wavelength of electromagnetic radiation may be appropriatefor use with the present techniques. Detector 140 may be chosen to bespecifically sensitive to the chosen targeted energy spectrum of lightsource 130.

In some embodiments, detector 140 may be configured to detect theintensity of light at the Red and IR wavelengths. In some embodiments,an array of sensors may be used and each sensor in the array may beconfigured to detect an intensity of a single wavelength. In operation,light may enter detector 140 after passing through the subject's tissue.Detector 140 may convert the intensity of the received light into anelectrical signal. The light intensity may be directly related to theabsorbance and/or reflectance of light in the tissue. That is, when morelight at a certain wavelength is absorbed or reflected, less light ofthat wavelength is received from the tissue by detector 140. Afterconverting the received light to an electrical signal, detector 140 maysend the detection signal to monitor 104, where the detection signal maybe processed and physiological parameters may be determined (e.g., basedon the absorption of the Red and IR wavelengths in the subject'stissue). In some embodiments, the detection signal may be preprocessedby sensor 102 before being transmitted to monitor 104.

In the embodiment shown, monitor 104 includes control circuitry 110,light drive circuitry 120, front end processing circuitry 150, back endprocessing circuitry 170, user interface 180, and communicationinterface 190. Monitor 104 may be communicatively coupled to sensor 102.

Control circuitry 110 may be coupled to light drive circuitry 120, frontend processing circuitry 150, and back end processing circuitry 170, andmay be configured to control the operation of these components. In someembodiments, control circuitry 110 may be configured to provide timingcontrol signals to coordinate their operation. For example, light drivecircuitry 120 may generate a light drive signal, which may be used toturn on and off the light source 130, based on the timing controlsignals. The front end processing circuitry 150 may use the timingcontrol signals to operate synchronously with light drive circuitry 120.For example, front end processing circuitry 150 may synchronize theoperation of an analog-to-digital converter and a demultiplexer with thelight drive signal based on the timing control signals. In addition, theback end processing circuitry 170 may use the timing control signals tocoordinate its operation with front end processing circuitry 150.

Light drive circuitry 120, as discussed above, may be configured togenerate a light drive signal that is provided to light source 130 ofsensor 102. The light drive signal may, for example, control theintensity of light source 130 and the timing of when light source 130 isturned on and off. When light source 130 is configured to emit two ormore wavelengths of light, the light drive signal may be configured tocontrol the operation of each wavelength of light. The light drivesignal may comprise a single signal or may comprise multiple signals(e.g., one signal for each wavelength of light). An illustrative lightdrive signal is shown in FIG. 2A.

FIG. 2A shows an illustrative plot of a light drive signal including redlight “on” period 202 and IR light “on period” 204 in accordance withsome embodiments of the present disclosure. Light “on” periods 202, and204 may be generated by light drive circuitry 120 under the control ofcontrol circuitry 110. As used herein, “on” and “off” may refer toswitching power or other components, high and low output states, highand low values within a continuous modulation, high and low duty cycles,other suitable relatively distinct states, or any combination thereof.The light drive signal may be provided to light source 130, includingred “on” period 202 and IR “on” period 204 to drive red and IR lightemitters, respectively, within light source 130. Red “on” period 202 mayhave a higher amplitude than IR “on” period 204 since red LEDs may beless efficient than IR LEDs at converting electrical energy into lightenergy. Additionally, red light may be absorbed and scattered more thanIR light when passing through perfused tissue at certain oxygensaturations. When the red and IR light sources are driven in this mannerthey emit pulses of light at their respective wavelengths into thetissue of a subject in order generate physiological signals thatphysiological monitoring system 100 may process to calculatephysiological parameters. It will be understood that the light driveamplitudes of FIG. 2A are merely exemplary, and that any suitableamplitudes or combination of amplitudes may be used, and may be based onthe light sources, the subject tissue, the determined physiologicalparameter, modulation techniques, power sources, any other suitablecriteria, or any combination thereof.

The light drive signal of FIG. 2A may also include “off” periods 220between the Red and IR light “on” periods. “Off” periods 220 are periodsduring which no drive current may be applied to light source 130. “Off”periods 220 may be provided, for example, to prevent overlap of theemitted light, since light source 130 may require time to turncompletely on and completely off. The period from time 216 to time 218may be referred to as a drive cycle, which includes four segments: a Redlight “on” periods 202, followed by an “off” period 220, followed by anIR light “on” period 204, and followed by an “off” period 220. Aftertime 218 the drive cycle may be repeated (e.g., as long as a light drivesignal is provided to light source 130). It will be understood that thestarting point of the drive cycle is merely illustrative and that thedrive cycle can start at any location within FIG. 2A, provided the cyclespans two light “on” periods and two “off” periods. Thus, each Red light“on” period 202 and each IR “on” period 204 may be understood to besurrounded by two dark periods 220.

Referring back to FIG. 1, front end processing circuitry 150 may receivea detection signal from detector 140 and provide one or more processedsignals to back end processing circuitry 170. The term “detectionsignal,” as used herein, may refer to any of the signals generatedwithin front end processing circuitry 150 as it processes the outputsignal of detector 140. Front end processing circuitry 150 may performvarious analog and digital processing of the detector signal. Onesuitable detector signal that may be received by front end processingcircuitry 150 is shown in FIG. 2B.

FIG. 2B shows an illustrative plot of detector signal 214 that may begenerated by a sensor in accordance with some embodiments of the presentdisclosure. The peaks of detector current waveform 214 may representcurrent signals provided by a detector, such as detector 140 of FIG. 1,when light is being emitted from a light source. The amplitude ofdetector current waveform 214 may be proportional to the light incidentupon the detector. The peaks of detector current waveform 214 may besynchronous with light “on” periods driving one or more emitters of alight source, such as light source 130 of FIG. 1. For example, detectorcurrent waveform 214 may be generated in response to a light sourcebeing driven by the light drive signal of FIG. 2A. The valleys ofdetector current waveform 214 may be synchronous with periods of timeduring which no light is being emitted by the light source. While nolight is being emitted by a light source during the valleys, detectorcurrent waveform 214 may not fall all of the way to zero. Rather, darkcurrent 222 may be present in the detector waveform. Since dark current222 may interfere with accurate determinations of physiologicalcharacteristics, dark current 222 may be removed as discussed in moredetail below.

Referring back to FIG. 1, front end processing circuitry 150, which mayreceive a detection signal, such as detector current waveform 214, mayinclude analog conditioner 152, analog-to-digital converter 154,demultiplexer 156, digital conditioner 158, decimator/interpolator 160,and dark subtractor 162.

Analog conditioner 152 may perform any suitable analog conditioning ofthe detector signal. The conditioning performed may include any type offiltering (e.g., low pass, high pass, band pass, notch, or any othersuitable filtering), amplifying, performing an operation on the receivedsignal (e.g., taking a derivative, averaging), performing any othersuitable signal conditioning (e.g., converting a current signal to avoltage signal), or any combination thereof.

The conditioned analog signal may be processed by analog-to-digitalconverter 154, which may convert the conditioned analog signal into adigital signal. Analog-to-digital converter 154 may operate under thecontrol of control circuitry 110. Analog-to-digital converter 154 mayuse timing control signals from control circuitry 110 to determine whento sample the analog signal. Analog-to-digital converter 154 may be anysuitable type of analog-to-digital converter of sufficient resolution toenable a physiological monitor to accurately determine physiologicalparameters.

Demultiplexer 156 may operate on the analog or digital form of thedetector signal to separate out different components of the signal. Forexample, detector current waveform 214 of FIG. 2B includes a Redcomponent, an IR component, and at least one dark component.Demultiplexer 156 may operate on detector current waveform 214 of FIG.2B to generate a Red signal, an IR signal, a first dark signal (e.g.,corresponding to the dark component that occurs immediately after theRed component), and a second dark signal (e.g., corresponding to thedark component that occurs immediately after the IR component).Demultiplexer 156 may operate under the control of control circuitry110. For example, demultiplexer 156 may use timing control signals fromcontrol circuitry 110 to identify and separate out the differentcomponents of the detector signal.

Digital conditioner 158 may perform any suitable digital conditioning ofthe detector signal. The digital conditioner may include any type ofdigital filtering of the signal (e.g., low pass, high pass, band pass,notch, or any other suitable filtering), amplifying, performing anoperation on the signal, performing any other suitable digitalconditioning, or any combination thereof.

Decimator/interpolator 160 may decrease the number of samples in thedigital detector signal. For example, decimator/interpolator 160 maydecrease the number of samples by removing samples from the detectorsignal or replacing samples with a smaller number of samples. Thedecimation or interpolation operation may include or be followed byfiltering to smooth the output signal.

Dark subtractor 162 may operate on the digital signal. In someembodiments, dark subtractor 162 may subtract dark values from the Redand IR components to generate adjusted Red and IR signals. For example,dark subtractor 162 may determine a subtraction amount from the darksignal portion of the detection signal and subtract it from the peakportion of the detection signal in order to reduce the effect of thedark signal on the peak. For example, in reference to FIG. 2A, adetection signal peak corresponding to red “on” period 202 may beadjusted by determining the amount of dark signal during the “off”period 220 preceding red “on” period 202. The dark signal amountdetermined in this manner may be subtracted from the detector peakcorresponding to red “on” period 202. Alternatively, the “off” period220 after red “on” period 202 may be used to correct red “on” period 202rather than the “off” period 220 preceding it. Additionally, an averageof the “off” periods 220 before and after red “on” period 202 may beused.

The components of front end processing circuitry 150 are merelyillustrative and any suitable components and combinations of componentsmay be used to perform the front end processing operations.

The front end processing circuitry 150 may be configured to takeadvantage of the full dynamic range of analog-to-digital converter 154.This may be achieved by applying gain to the detection signal by analogconditioner 152 to map the expected range of the detection signal to thefull or close to full output range of analog-to-digital converter 154.The output value of analog-to-digital converter 154, as a function ofthe total analog gain applied to the detection signal, may be given as:ADC Value∝Total Analog Gain×[Ambient Light+LED Light].

Ideally, when ambient light is zero and when the light source is off,the analog-to-digital converter 154 will read just above the minimuminput value. When the light source is on, the total analog gain may beset such that the output of analog-to-digital converter 154 may readclose to the full scale of analog-to-digital converter 154 withoutsaturating. This may allow the full dynamic range of analog-to-digitalconverter 154 to be used for representing the detection signal, therebyincreasing the resolution of the converted signal. In some embodiments,the total analog gain may be reduced by a small amount so that smallchanges in the light level incident on the detector do not causesaturation of analog-to-digital converter 154.

However, if the contribution of ambient light is large relative to thecontribution of light from a light source, the total analog gain appliedto the detection current may need to be reduced to avoid saturatinganalog-to-digital converter 154. When the analog gain is reduced, theportion of the signal corresponding to the light source may map to asmaller number of analog-to-digital conversion bits. Thus, more ambientlight noise in the input of analog-to-digital converter 154 may resultsin fewer bits of resolution for the portion of the signal from the lightsource. This may have a detrimental effect on the signal-to-noise ratioof the detection signal. Accordingly, passive or active filtering orsignal modification techniques may be employed to reduce the effect ofambient light on the detection signal that is applied toanalog-to-digital converter 154, and thereby reduce the contribution ofthe noise component to the converted digital signal.

Back end processing circuitry 170 may include processor 172 and memory174. Processor 172 may be adapted to execute software, which may includean operating system and one or more applications, as part of performingthe functions described herein. Processor 172 may receive and processphysiological signals received from front end processing circuitry 150.For example, processor 172 may determine one or more physiologicalparameters based on the received physiological signals. Memory 174 mayinclude any suitable computer-readable media capable of storinginformation that can be interpreted by processor 172. This informationmay be data or may take the form of computer-executable instructions,such as software applications, that cause the microprocessor to performcertain functions and/or computer-implemented methods. Depending on theembodiment, such computer-readable media may include computer storagemedia and communication media. Computer storage media may includevolatile and non-volatile, removable and non-removable media implementedin any method or technology for storage of information such ascomputer-readable instructions, data structures, program modules orother data. Computer storage media may include, but is not limited to,RAM, ROM, EPROM, EEPROM, flash memory or other solid state memorytechnology, CD-ROM, DVD, or other optical storage, magnetic cassettes,magnetic tape, magnetic disk storage or other magnetic storage devices,or any other medium which can be used to store the desired informationand which can be accessed by components of the system. Back endprocessing circuitry 170 may be communicatively coupled with useinterface 180 and communication interface 190.

User interface 180 may include user input 182, display 184, and speaker186. User input 182 may include any type of user input device such as akeyboard, a mouse, a touch screen, buttons, switches, a microphone, ajoy stick, a touch pad, or any other suitable input device. The inputsreceived by user input 182 can include information about the subject,such as age, weight, height, diagnosis, medications, treatments, and soforth. In an embodiment, the subject may be a medical patient anddisplay 184 may exhibit a list of values which may generally apply tothe patient, such as, for example, age ranges or medication families,which the user may select using user input 182. Additionally, display184 may display, for example, an estimate of a subject's blood oxygensaturation generated by monitor 104 (referred to as an “SpO₂”measurement), pulse rate information, respiration rate information,blood pressure, any other parameters, and any combination thereof.Display 184 may include any type of display such as a cathode ray tubedisplay, a flat panel display such a liquid crystal display or plasmadisplay, or any other suitable display device. Speaker 186 within userinterface 180 may provide an audible sound that may be used in variousembodiments, such as for example, sounding an audible alarm in the eventthat a patient's physiological parameters are not within a predefinednormal range.

Communication interface 190 may enable monitor 104 to exchangeinformation with external devices. Communications interface 190 mayinclude any suitable hardware, software, or both, which may allowmonitor 104 to communicate with electronic circuitry, a device, anetwork, a server or other workstations, a display, or any combinationthereof. Communications interface 190 may include one or more receivers,transmitters, transceivers, antennas, plug-in connectors, ports,communications buses, communications protocols, device identificationprotocols, any other suitable hardware or software, or any combinationthereof. Communications interface 190 may be configured to allow wiredcommunication (e.g., using USB, RS-232 or other standards), wirelesscommunication (e.g., using WiFi, IR, WiMax, BLUETOOTH, UWB, or otherstandards), or both. For example, communications interface 190 may beconfigured using a universal serial bus (USB) protocol (e.g., USB 2.0,USB 3.0), and may be configured to couple to other devices (e.g., remotememory devices storing templates) using a four-pin USB standard Type-Aconnector (e.g., plug and/or socket) and cable. In some embodiments,communications interface 190 may include an internal bus such as, forexample, one or more slots for insertion of expansion cards.

It will be understood that the components of physiological monitoringsystem 100 that are shown and described as separate components are shownand described as such for illustrative purposes only. In someembodiments the functionality of some of the components may be combinedin a single component. For example, the functionality of front endprocessing circuitry 150 and back end processing circuitry 170 may becombined in a single processor system. Additionally, in some embodimentsthe functionality of some of the components of monitor 104 shown anddescribed herein may be divided over multiple components. For example,some or all of the functionality of control circuitry 110 may beperformed in front end processing circuitry 150, in back end processingcircuitry 170, or both. In other embodiments, the functionality of oneor more of the components may be performed in a different order or maynot be required. In an embodiment, all of the components ofphysiological monitoring system 100 can be realized in processorcircuitry.

FIG. 2C shows illustrative timing diagrams of drive cycle modulation andcardiac cycle modulation in accordance with some embodiments of thepresent disclosure. Plot 250 may include a timing diagram of anexemplary cardiac cycle modulation, including red light modulation 252and IR light modulation 254. In the embodiment illustrated in plot 250,the IR light remains at a constant level and the red light is “on” onlyduring the diastole period. The period of the modulation cycle maycorrespond to time interval 260. In a further embodiment, the system mayreplace some or all of the “off” periods with “on” periods of lowerlight intensity, shorter duty cycles, any other suitable parametervariations, or a combination thereof. It will be understood that theaforementioned cardiac cycle modulation technique is merely exemplaryand that the system may use any suitable cardiac cycle modulationtechnique.

Region 256 of plot 250 indicates an interval of the timing diagram whereboth red light modulation 252 and IR light modulation 254 are in an “on”portion of the cardiac cycle modulation. Plot 270 shows an illustrativeportion of region 256, where the system is employing a cardiac cyclemodulation in addition to the drive cycle modulation. Plot 250 mayinclude a drive cycle modulation technique with a period of timeinterval 272. The time scale of plot 270 may be significantly shorterthan the time scale of plot 250, such that time interval 272 issignificantly shorter than time interval 260. For example, time interval260 (i.e., the period of the cardiac cycle modulation) may be on theorder of 1 second, while time interval 272 (i.e., the period of thedrive cycle modulation) may be on the order of 1 ms. Time interval 272may include a sequence of red “on” portion 274, a first “off” portion276, IR “on” portion 278, and a second “off” portion 280. The first“off” portion 276 and second “off” portion 280 may be used to determinethe level of ambient light, noise, dark current, other suitable signals,or any combination thereof. The system may subtract the background ordark level from the levels received during red “on” portion 274 and IR“on” period 278.

Region 258 of plot 250 indicates an interval of the timing diagram wherethe red light modulation 252 is in an “off” portion of the cardiac cyclemodulation and IR light modulation 254 is in an “on” portion of thecardiac cycle modulation. Plot 282 shows an illustrative portion ofregion 258, where the system is employing a drive cycle modulationtechnique in addition to the cardiac cycle modulation. Plot 282 mayinclude a drive cycle modulation technique with a period of timeinterval 284. The time scale of plot 282 may be significantly shorterthan the time scale of plot 250, such that time interval 284 issignificantly shorter than time interval 260. In some embodiments, thetime scale of plot 282 may be the same as the time scale of plot 270.Time interval 284 may include a sequence of red “on” portion 286, afirst “off” portion 288, IR “on” portion 290, and a second “off” portion292. The red “on” portion 286 may include less red light emitted thanduring red “on” portion 274, or no red light emitted, as red lightmodulation 252 is in an “off” phase during region 258. The first “off”portion 288 and second “off” portion 292 may be used to determine thelevel of ambient light, noise, dark current, other suitable signals, orany combination thereof. The system may subtract the background or darklevel from the levels received during red “on” portion 286 and IR “on”portion 290.

Red light modulation 252 may be in an “on” portion during region 256(illustrated in detail in plot 270) and an “off” portion during region258 (illustrated in detail in plot 282). Thus, the level of red lightindicated by red “on” portion 274 is at a high level and the level ofred light indicated by red “on” portion 286 is at a low level. This isillustrative of an embodiment where drive cycle modulation occurstogether with cardiac cycle modulation. It will be understood that thetechniques illustrated by FIG. 2C are merely exemplary and that othersuitable techniques may be used for drive cycle modulation, as describedabove. It will also be understood that other suitable methods may beused to combine the drive cycle modulation and the cardiac cyclemodulation. It will also be understood that the combining technique maydepend in part on the particular drive cycle modulation and cardiaccycle modulation technique. It will also be understood that conventionalservo algorithms may be used in addition to combinations of drive cyclemodulation and cardiac cycle modulation.

FIG. 3 is a perspective view of an embodiment of a physiologicalmonitoring system 310 in accordance with some embodiments of the presentdisclosure. In some embodiments, one or more components of physiologicalmonitoring system 310 may include one or more components ofphysiological monitoring system 100 of FIG. 1. System 310 may includesensor unit 312 and monitor 314. In some embodiments, sensor unit 312may be part of an oximeter. Sensor unit 312 may include one or morelight source 316 for emitting light at one or more wavelengths into asubject's tissue. One or more detector 318 may also be provided insensor unit 312 for detecting the light that is reflected by or hastraveled through the subject's tissue. Any suitable configuration oflight source 316 and detector 318 may be used. In an embodiment, sensorunit 312 may include multiple light sources and detectors, which may bespaced apart. System 310 may also include one or more additional sensorunits (not shown) that may, for example, take the form of any of theembodiments described herein with reference to sensor unit 312. Anadditional sensor unit may be the same type of sensor unit as sensorunit 312, or a different sensor unit type than sensor unit 312 (e.g., aphotoacoustic sensor). Multiple sensor units may be capable of beingpositioned at two different locations on a subjects body.

In some embodiments, sensor unit 312 may be connected to monitor 314 asshown. Sensor unit 312 may be powered by an internal power source, e.g.,a battery (not shown). Sensor unit 312 may draw power from monitor 314.In another embodiment, the sensor may be wirelessly connected to monitor314 (not shown). Monitor 314 may be configured to calculatephysiological parameters based at least in part on data relating tolight emission and acoustic detection received from one or more sensorunits such as sensor unit 312. For example, monitor 314 may beconfigured to determine pulse rate, blood pressure, blood oxygensaturation (e.g., arterial, venous, or both), hemoglobin concentration(e.g., oxygenated, deoxygenated, and/or total), any other suitablephysiological parameters, or any combination thereof. In someembodiments, calculations may be performed on the sensor units or anintermediate device and the result of the calculations may be passed tomonitor 314. Further, monitor 314 may include display 320 configured todisplay the physiological parameters or other information about thesystem. In the embodiment shown, monitor 314 may also include a speaker322 to provide an audible sound that may be used in various otherembodiments, such as for example, sounding an audible alarm in the eventthat a subject's physiological parameters are not within a predefinednormal range. In some embodiments, the system 310 includes a stand-alonemonitor in communication with the monitor 314 via a cable or a wirelessnetwork link. In some embodiments, monitor 314 may be implemented asmonitor 104 of FIG. 1.

In some embodiments, sensor unit 312 may be communicatively coupled tomonitor 314 via a cable 324. Cable 324 may include electronic conductors(e.g., wires for transmitting electronic signals from detector 318),optical fibers (e.g., multi-mode or single-mode fibers for transmittingemitted light from light source 316), any other suitable components, anysuitable insulation or sheathing, or any combination thereof. In someembodiments, a wireless transmission device (not shown) or the like maybe used instead of or in addition to cable 324. Monitor 314 may includea sensor interface configured to receive physiological signals fromsensor unit 312, provide signals and power to sensor unit 312, orotherwise communicate with sensor unit 312. The sensor interface mayinclude any suitable hardware, software, or both, which may be allowcommunication between monitor 314 and sensor unit 312.

Calibration device 380, which may be powered by monitor 314, a battery,or by a conventional power source such as a wall outlet, may include anysuitable calibration device. Calibration device 380 may becommunicatively coupled to monitor 314 via communicative coupling 382,and/or may communicate wirelessly (not shown). In some embodiments,calibration device 380 is completely integrated within monitor 314. Insome embodiments, calibration device 380 may include a manual inputdevice (not shown) used by an operator to manually input referencesignal measurements obtained from some other source (e.g., an externalinvasive or non-invasive physiological measurement system).

In the illustrated embodiment, system 310 includes a multi-parameterphysiological monitor 326. The monitor 326 may include a cathode raytube display, a flat panel display (as shown) such as a liquid crystaldisplay (LCD) or a plasma display, or may include any other type ofmonitor now known or later developed. Multi-parameter physiologicalmonitor 326 may be configured to calculate physiological parameters andto provide a display 328 for information from monitor 314 and from othermedical monitoring devices or systems (not shown). For example,multi-parameter physiological monitor 326 may be configured to displayan estimate of a subject's blood oxygen saturation and hemoglobinconcentration generated by monitor 314. Multi-parameter physiologicalmonitor 326 may include a speaker 330.

Monitor 314 may be communicatively coupled to multi-parameterphysiological monitor 326 via a cable 332 or 334 that is coupled to asensor input port or a digital communications port, respectively and/ormay communicate wirelessly (not shown). In addition, monitor 314 and/ormulti-parameter physiological monitor 326 may be coupled to a network toenable the sharing of information with servers or other workstations(not shown). Monitor 314 may be powered by a battery (not shown) or by aconventional power source such as a wall outlet.

In some embodiments, all or some of monitor 314 and multi-parameterphysiological monitor 326 may be referred to collectively as processingequipment. In some embodiments, any of the processing components and/orcircuits, or portions thereof, of FIGS. 1 and 3 may be referred tocollectively as processing equipment. For example, processing equipmentmay be configured to generate light drive signals, amplify, filter,sample and digitize detector signals, and calculate physiologicalinformation from the digitized signal. In some embodiments, all or someof the components of the processing equipment may be referred to as aprocessing module.

FIG. 4 is flow diagram 400 showing illustrative steps for determining aphysiological parameter in accordance with some embodiments of thepresent disclosure. In some embodiments, the system may emit a photonicsignal from a first light source and use information from the relatedattenuated signal to generate a light drive signal for a second lightsource.

In step 402, the system may generate a first light drive signal. Thelight drive signal may be used by a light source to emit a photonicsignal. The light source may be one or more LEDs, laser diodes, othersuitable device, or any combination thereof. For example, the lightsource may include light source 130 of FIG. 1 or light source 316 ofFIG. 3. In some embodiments, the light source may include LEDs ofmultiple wavelengths, for example, a red LED and an IR led. In someembodiments, the light source may include multiple LEDs of the samewavelength, multiple LEDs of different wavelengths, any other suitablearrangement, or any combination thereof. In some embodiments, the lightsource may include a fiber optic or other light pipe to communicatelight from one location to another. In some embodiments, the light drivesignal may include or be a component of a cardiac cycle modulation. Forexample, the first light drive signal may be configured to activate oneLED to emit a photonic signal and not activate other LEDs, such thatsome physiological parameters may be determined, but with lower powerconsumption than when the other LEDs are illuminated.

In step 404, the system may receive a light signal. The light may bereceived using a sensor, for example, detector 140 of FIG. 1 or detector318 of FIG. 3. The light signal may be attenuated by the subject. Thereceived light signal may in part include light from the first photonicsignal. For example, the system may emit light that is reflected by thesubject or transmitted through the subject. The interaction of theemitted light with the subject may cause the light to become attenuated.In some embodiments, the attenuation of the light may depend on thewavelength of the light and the tissue with which the light interacts.For example, particular wavelengths of light may be attenuated morestrongly by oxyhemoglobin than other wavelengths. In some embodiments,the system may amplify the received signal using front end processorcircuitry. In some embodiments, the gain may be modulated using atechnique correlated to the cardiac cycle modulation. The gain of theamplifier may be adjusted based on the emitted light brightness,historical information related to the brightness of prior receivedattenuated signals, other suitable information, or any combinationthereof, so that the amplified signal matches the range of theanalog-to-digital converter and thus increases resolution. In someembodiments, the system may account for the gain using hardware,software, or any combination thereof, such that the original intensityinformation is retained.

In step 406, the system may analyze the received light signal todetermine when to activate a second light source and/or parameters of asecond photonic signal. In some embodiments, the second light source mayinclude one or more emitters. In some embodiments, the system mayidentify peaks, valleys, inflection points, slope changes, fiducialpoints, other suitable elements, or any combination thereof in thereceived light source. In some embodiments, the system may useinformation determined from analyzing the first light source in additionto other information. Other information may include, for example,historical analysis of prior cardiac cycles and information fromexternal sensors. For example, the system may determine an average pulseperiod from a number of prior pulse cycles. Statistical information suchas the standard deviation may also be calculated to in part determine aconfidence parameter for the historical information. In someembodiments, the system may use a respiration rate. For example, thesystem may determine a respiration rate from an external sensor and useinformation from the respiration rate to determine a modulationtechnique. In some embodiments, the cardiac cycle modulation applied tothe second light drive signal may be varied based on the historical andstatistical information. In some embodiments, the system may determineto turn on a second light source with a time offset to the element ofinterest in the cardiac cycle. For example, the system may turn on thesecond light source a certain number of milliseconds prior to the peak(or expected peak) of a pulse signal. In a further example, the systemmay wait a certain number of milliseconds following a peak in an ECGsignal. In some embodiments, the time offsets may be adjusted based onprior signal analysis, adjusted by user input, adjusted by predeterminedvalues, adjusted by any other suitable technique, or any combinationthereof. In some embodiments, parameters of the second photonic signal,for example duty cycle, decimation ratio, and brightness, may bedetermined based on analysis of the received light signal.

In step 408, the system may generate a second light drive signal. Thesecond light drive signal may be configured to activate a second lightsource or a different emitter from the first light source to emit asecond photonic signal. For example, in a cardiac cycle modulation wherea first light source remains on at a constant level (e.g., in a drivecycle modulation) and a second light source is turned on and off tooptimize power consumption, the second light drive signal may cause asecond light source to emit light at a particular time or times in thecardiac cycle. Light from the second photonic signal may be attenuatedby the subject and received by a sensor. In some embodiments, the systemmay adjust the second light drive signal based on historical data. Forexample, the system may use information from prior pulse cycles todetermine an optimal emitter brightness.

In step 410, the system may determine a physiological parameter usinginformation from the attenuated photonic signals. The physiologicalparameter may be determined using any suitable hardware technique,software technique, or combination thereof. In some embodiments,processing equipment remote to the system may be used to determinephysiological parameters. The system may display the determinedphysiological parameter using a local display (e.g., display 320 of FIG.3 or display 328 of FIG. 3), display them on a remote display, publishthe data to a server or website, make the parameters available to a userby any other suitable technique, or any combination thereof.

FIG. 5 shows illustrative timing diagram 500 of a physiologicalmonitoring system in accordance with some embodiments of the presentdisclosure. In some embodiments, the system may use a first light drivesignal to identify the systole periods of the cardiac cycle and modulatea second light drive signal to increase light intensity concurrent withthe systole periods. Timing diagram 500 may include time on the abscissaaxis and either arbitrary amplitude or unitless dimensions on theordinate axis depending on the row of the diagram. Timing diagram 500may include time periods related to the cardiac cycle, including systoleperiod 502, diastole period 504, systole period 506, and diastole period508. Timing diagram 500 may also include diastole period 510, thoughonly a portion of this diastole period is drawn. Timing diagram 500 mayinclude PPG signal 552, ECG signal 554, red light drive signal 556, andIR light drive signal 558. PPG signal 552 and ECG signal 554 are shownwith arbitrary units on the ordinate axis. Red light drive signal 556and IR light drive signal 558 are shown as “on” or “off” states withoutunits associated with the ordinate axis.

Timing diagram 500 may include ECG signal 554, which is indicative ofelectrical signals associated with the cardiac cycle. ECG signal 554 mayinclude P wave 522, Q wave 524, R wave 526, S wave 528, and T wave 530.In some embodiments, ECG signals may be used in addition to or in placeof information from PPG signal 552.

It will be understood that the particular alignment of ECG signal 554with elements in PPG signal 552 is dependent upon the location on thesubject where the PPG signal is measured. For example, a PPG signal istypically monitored at a location remote from the heart, creating a timedelay between the ECG signal and corresponding pulses in the PPG.Additionally, there may be a time delay between the electricaldepolarization of the heart (QRS complex of an ECG) and the ejection ofblood from the heart. This time delay may include an electro-mechanicaldelay before the heart muscle contracts and a period of isovolumetriccontraction where the muscle tension in the heart increases but thepressure does not exceed the aortic pressure. During the period ofisovolumetric contraction, the aortic valve may remain closed.

In some embodiments, the system may send a drive signal without acardiac modulation to a first light source. For example, the system maynot apply a cardiac cycle modulation to IR light drive signal 558. IRlight drive signal 558 may, for example, correspond to the first lightdrive signal generated at step 402 of FIG. 4. In some embodiments, thesystem may detect an attenuated photonic signal associated with IR lightdrive signal 558 throughout the cardiac pulse cycle. In someembodiments, elements of the cardiac cycle may be identified using othersignals, for example, ECG signal 554. The system may determine periodsof the cardiac cycle and apply a cardiac cycle modulation to a secondlight source. For example, red light drive signal 556 may be switched onat period 532 during systole period 502, off during diastole period 504,on at period 534 during systole period 506, and off during diastoleperiod 508. Red light drive signal 556 may, for example, correspond tothe second light drive signal generated at step 408 of FIG. 4. Thus, thecardiac cycle modulation applied to red light drive signal 556 may besubstantially synchronous with the systole periods of the cardiac cycle.In some embodiments, the system may determine the timing of the systoleperiods using information from the attenuated first photonic signalassociated with IR light drive signal 558. In some embodiments, thesystem may use historical information from multiple cardiac cycles todetermine the red light drive signal 556.

It will be understood, and illustrated in the following figures, thatthe cardiac cycle modulation shown in timing diagram 500 is merelyillustrative and that the system may use other suitable modulationtechniques. It will also be understood, for this figure and thefollowing figures, that the system may use a drive cycle modulationtechnique as illustrated in FIG. 2C and conventional servo algorithms inaddition to the cardiac cycle modulation. It will also be understood,for this figure and the following figures, that the alignment of PPGsignal 552, ECG signal 554, red light drive signal 556 and IR lightdrive signal 558 is merely exemplary and may include various timeoffsets and adjustments (not shown) dependent upon placement of sensorson the subject, physiological conditions, emitter equipment, sensorequipment, other suitable reasons, or any combination thereof. In someembodiments, the system may not include IR light drive signal 558 andmay apply a cardiac cycle modulation technique to all light drivesignals, as will be illustrated in FIGS. 10-16.

FIG. 6 shows another illustrative timing diagram 600 of a physiologicalmonitoring system in accordance with some embodiments of the presentdisclosure. In some embodiments, the system may use a first light drivesignal to identify diastole period periods in the cardiac cycle andmodulate a second light drive signal to increase light intensityconcurrent with the diastole period periods. Timing diagram 600 mayinclude time periods related to the cardiac cycle, including systoleperiod 602, diastole period 604, systole period 606, and diastole period608. Timing diagram 600 may include PPG signal 610, ECG signal 612, redlight drive signal 614, and IR light drive signal 616. PPG signal 610and ECG signal 612 are shown with arbitrary units on the ordinate axis.Red light drive signal 614 and IR light drive signal 616 are shown as“on” or “off” portions without units associated with the ordinate axis.

In some embodiments, the system may send a drive signal without acardiac cycle modulation to a first light source. For example, thesystem may generate IR light drive signal 616 without cardiac cyclemodulation. IR light drive signal 616 may, for example, be the firstlight drive signal of step 402 of FIG. 4. In some embodiments, thesystem may detect an attenuated photonic signal associated with IR lightdrive signal 616 throughout the cardiac cycle. In some embodiments,elements of the cardiac cycle may be identified using other signals, forexample, ECG signal 612. The system may determine periods of the cardiaccycle and apply a cardiac cycle modulation to a second light source. Forexample, red light drive signal 614 may be switched off during systoleperiod 602, on at period 618 during diastole period 604, off duringsystole period 606, and on at period 620 during diastole period 608. Redlight drive signal 614 may, for example, correspond to the second lightdrive signal generated at step 408 of FIG. 4. Thus, the cardiac cyclemodulation applied to red light drive signal 614 may be substantiallysynchronous with the diastole periods of the cardiac cycle.

FIG. 7 shows another illustrative timing diagram 700 of a physiologicalmonitoring system in accordance with some embodiments of the presentdisclosure. In some embodiments, the system may use a first light drivesignal to identify a dicrotic notch or other point of interest in thecardiac cycle and modulate a second light drive signal to increase lightintensity concurrent with the dicrotic notch. Timing diagram 700 mayinclude ECG signal 710, PPG signal 714, red light drive signal 716, andIR light drive signal 718. ECG signal 710 and PPG signal 714 are shownwith arbitrary units on the ordinate axis. PPG signal 714 is shown as alight intensity signal where a higher amplitude is indicative of morereceived light, or less blood in the sampled tissue. In someembodiments, PPG signal 714 in timing diagram 700 may be illustrated ina manner inverted from the common technique of illustrating PPG signals.Accordingly, elements of PPG signal 714 labeled herein as, for example,peaks and troughs, may be inverted from certain medical conventions. Redlight drive signal 716 and IR light drive signal 718 are shown as “on”or “off” states without units associated with the ordinate axis.

In some embodiments, the system may send a drive signal to a first lightsource without cardiac modulation. For example, the system may not applya cardiac cycle modulation to IR light drive signal 718. IR light drivesignal 718 may be the first light drive signal of step 402 of FIG. 4. Insome embodiments, the system may detect an attenuated photonic signalassociated with IR light drive signal 718 throughout the cardiac pulsecycle. In some embodiments, elements of the cardiac cycle may beidentified using other signals, for example, ECG signal 710. The systemmay determine periods of the cardiac cycle and apply a cardiac cyclemodulation to a second light source. For example, the system mayidentify notch 720 in PPG signal 714. In some embodiments, the systemmay identify notch 720 indirectly using an element of PPG signal 714(e.g., trough 724), using elements of ECG signal 710, by any othersuitable technique, or any combination thereof. For example, notch 720may be identified using a time offset from an element of ECG signal 710.As illustrated, there may be a time delay between trough 724 and notch720. For example, red light drive signal 716 may be switched on duringperiod 726, substantially concurrent with notch 720, and on duringperiod 728, substantially concurrent with notch 722. Red light drivesignal 716 may be the second light drive signal of step 408 of FIG. 4.Thus, the cardiac cycle modulation applied to red light drive signal 716is substantially synchronous with the dicrotic notch of the cardiaccycle. In some embodiments, the system may turn on a light source beforeperiod 726 or a desired point of interest and turn off a light sourcefollowing period 726 or a point of interest so that the photonic signalcan stabilize, so that the detector can stabilize, so that theprocessing equipment can obtain extra samples for averaging,interpolating, or decimating, for amplifier gain adjustments tostabilize, for any other suitable reason, or any combination thereof.The system may determine the time offsets between the “on” portions andthe region of interest based on user input, predetermined parameters,previous measurements, other suitable parameters, or any combinationthereof. It will be understood that the use of a dicrotic notch fornotch 720 is merely exemplary and that the system may identify anysuitable point of interest. For example, the system may identifyfiducial points such as local maxima and minima. Light sources may beilluminated, for example, as described below in reference to elements ofFIG. 25.

FIG. 8A shows another illustrative timing diagram 800 of a physiologicalmonitoring system in accordance with some embodiments of the presentdisclosure. In some embodiments, the system may use a first light drivesignal to identify the peak and troughs of the PPG signal and modulate asecond light drive signal to increase light intensity concurrent withthe PPG peaks. Timing diagram 800 may include PPG signal 802, red lightdrive signal 804, IR light drive signal 806, and ECG signal 818. PPGsignal 802 and ECG signal 818 are shown with arbitrary units on theordinate axis. Red light drive signal 804 and IR light drive signal 806are shown as “on” or “off” states without units associated with theordinate axis.

In some embodiments, the system may send a light drive signal to a firstlight source without a cardiac cycle modulation. For example, IR lightdrive signal 806 may not include a cardiac cycle modulation. IR lightdrive signal 806 may be the first light drive signal of step 402 of FIG.4. In some embodiments, the system may detect an attenuated photonicsignal associated with IR light drive signal 806 throughout the cardiacpulse cycle. In some embodiments, peaks, troughs, and other elements ofthe PPG signal or cardiac may be identified using other signals, forexample, an ECG signal. The system may determine periods of the cardiaccycle and apply a cardiac cycle modulation to a second light source. Forexample, the system may identify point 808 and point 810 in PPG signal802. In some embodiments, point 808 and 810 may represent peaks ortroughs, depending on the orientation of the PPG signal. The system mayturn on red light drive signal 804 during period 812, substantiallyconcurrent with peak 808, and during period 816, substantiallyconcurrent with peak 810. Red light drive signal 804 may be the secondlight drive signal of step 408 of FIG. 4. Thus, the cardiac cyclemodulation applied to red light drive signal 804 is substantiallysynchronous with the peak of the PPG signal. In some embodiments, thesystem may turn on a light source before peaks 808 and 810 and turn offa light source following peaks 808 and 810 so that the photonic signalcan stabilize, so that the detector can stabilize, so that theprocessing equipment can obtain extra samples for averaging,interpolating, or decimating, for amplifier gain adjustments tostabilize, for any other suitable reason, or any combination thereof.The system may determine the time offsets between the “on” periods andthe region of interest based on user input, predetermined parameters,previous measurements, other suitable parameters, or any combinationthereof.

FIG. 8B shows another illustrative timing diagram 820 of a physiologicalmonitoring system in accordance with some embodiments of the presentdisclosure. As shown in FIG. 8A, the system may use a first light drivesignal to identify the peak and troughs of the PPG signal and modulate asecond light drive signal to increase light intensity concurrent withthe PPG peaks. In some embodiments, the use of the first and secondlight drive signals may be selected based on the blood oxygensaturation, other physiological parameters, other system parameters, orany combination thereof.

Timing diagram 820 may include SpO₂ signal 822, PPG signal 836, redlight drive signal 838, and IR light drive signal 840. SpO₂ signal 822may be shown with units of percentage on the ordinate axis. PPG signal836 may be shown with arbitrary units on the ordinate axis. Red lightdrive signal 838 and IR light drive signal 840 may be shown as “on” or“off” states without units associated with the ordinate axis.

In some embodiments, the blood oxygen saturation may be compared to athreshold or target value, such as threshold 830. For example, threshold830 may be approximately 70-80% of maximum blood oxygen saturation.During time interval 824, when the SpO₂ signal is greater than threshold830, the system may operate in a first mode. For example, asillustrated, the system may use IR light drive signal 840 to monitorcardiac activity throughout a cardiac pulse cycle, and may modulate redlight drive signal 838 accordingly. At time point 826, SpO₂ signal maycross threshold 830 at point 834. In some embodiments, this may beindicative of decreasing blood oxygen saturation. During time interval828 following time point 826, the system may operate in a second mode.For example, as illustrated, the system may use red light drive signal838 to monitor cardiac activity throughout a cardiac pulse cycle, andmay modulate IR light drive signal 840 accordingly.

In some embodiments, blood with a relatively high blood oxygensaturation may absorb IR light more strongly than red light. Thus, theIR light may be more sensitive to pulsatile signals than red light. Forexample, blood with a high blood oxygen saturation may have a relativelyhigher concentration of oxyhemoglobin and a relatively lowerconcentration of deoxyhemoglobin, resulting in higher IR sensitivity.Conversely, in some embodiments, blood with a relatively low bloodoxygen saturation may absorb red light more strongly than infraredlight. For example, blood with a low blood oxygen saturation may have arelatively lower concentration of oxyhemoglobin and a relatively higherconcentration of deoxyhemoglobin, resulting in higher red sensitivity.Thus, it may be desired to monitor activity with the more sensitiveavailable wavelength.

It will be understood that selection and/or switching of the first andsecond light sources may depend on the blood oxygen saturation as wellas additional physiological parameters and system parameters. Forexample, a red wavelength LED may consume more power than an IR LED, andthus it may be desired to use the lowest power consuming techniquepossible. In some embodiments, the system may select the light sourcecapable of detecting the greatest pulsatile amplitude per unit of powerconsumed. In some other embodiments, the system may select the lightsource capable of detecting the greatest pulsatile amplitude. It willalso be understood that in some embodiments, a decreasing blood oxygensaturation or other physiological parameter may result in the system notusing a cardiac cycle modulation technique in order to attain thehighest possible quality physiological information. This will bediscussed below, for example with relation to FIG. 21.

FIG. 9 is flow diagram 900 showing illustrative steps for determining aphysiological parameter in accordance with some embodiments of thepresent disclosure. In some embodiments, the system may emit a photonicsignal correlated to physiological pulses from a light source, and useinformation from the related attenuated signal to determinephysiological parameters. In some embodiments, varying parameters of thelight drive signal may reduce, optimize, or otherwise suitably alter thepower consumption of the system.

In step 902, the system may generate a light drive signal, in partcorrelated to physiological pulses. The system may generate a lightdrive signal used by a light source to emit a photonic signal. The lightsource may be one or more emitters of one or more wavelengths, and theymay emit one or more photonic signals. For example, the light source mayinclude light source 130 of FIG. 1 or light source 316 of FIG. 3. Insome embodiments, the light source may include LEDs of multiplewavelengths, for example, a red LED and an IR LED. In some embodiments,the light source may include multiple LEDs of the same wavelength,multiple LEDs of different wavelengths, any other suitable arrangement,or any combination thereof. In some embodiments, the light source mayinclude a fiber optic or other light pipe to communicate light from onelocation to another.

The system may generate the light drive signal such that a parameter ofthe emitted one or more photonic signals varies substantiallysynchronously with physiological pulses of the subject. For example, thesystem may generate a light drive signal that varies with a period thesame as or closely related to the period of the cardiac cycle, thusgenerating a cardiac cycle modulation. The system may vary parametersrelated to the light drive signal including drive current or lightbrightness, duty cycle, firing rate, modulation parameters, othersuitable parameters, or any combination thereof. In some embodiments,the system may use a cardiac cycle modulation that spans several cardiaccycles (e.g., emitting light from a light source during the first one ofevery five cycles). In some embodiments, the system may generate a lightdrive signal that modulates parameters of more than one light sourceusing more than one modulation technique. It will be understood that thesystem may apply this cardiac cycle modulation to the light drive signalin addition to a drive cycle modulation, as illustrated in FIG. 2C, andconventional servo algorithms.

In some embodiments, physiological pulses may be cardiac pulses,respiratory pulses, muscular pulses, any other suitable pulses, or anycombination thereof. Where physiological pulses are respiratory pulses,they may relate to respiration rate, inspiration, expiration, ventilatorparameters, changes in a respiratory pressure signals, any othersuitable parameter, or any combination thereof. In some embodiments,particular segments of a respiratory cycle may provide an increasedsignal to noise ratio, increased signal strength, increasedphysiological parameter accuracy, increased physiological parameterprecision, any other suitable parameters, or any combination thereof. Insome embodiments, respiration may cause variations inphotoplethysmography data, and thus it may be desired to correlate amodulation technique with respiration variations or both respirationvariations and cardiac pulses.

In some embodiments, the system may use information to determine thecardiac cycle modulation. Information may include, for example,historical analysis of prior cardiac cycles and information fromexternal sensors. For example, the system may determine an average pulseperiod from a number of prior pulse cycles. Statistical information suchas the standard deviation may also be calculated to in part determine aconfidence parameter for the historical information. In someembodiments, the cardiac cycle modulation applied to the light drivesignal may be varied based on the historical and statisticalinformation.

In step 904, the system may receive a light signal. The light may bereceived using a sensor, for example, detector 140 of FIG. 1 or detector318 of FIG. 3. The light signal may be attenuated by the subject. Thereceived light signal may in part include light from the photonicsignal. For example, the system may emit light that is reflected by thesubject or transmitted through the subject. Interactions of the emittedlight with the subject may cause the light to become attenuated. In someembodiments, the attenuation of the light may depend on the wavelengthof the light and the tissue with which the light interacts. For example,particular wavelengths of light may be attenuated more strongly byoxyhemoglobin than other wavelengths. In some embodiments, the systemmay amplify the received signal using front end processor circuitry. Thegain of the amplifier may be adjusted based on the emitted lightbrightness, historical information related to the brightness of priorreceived attenuated signals, other suitable information, or anycombination thereof, so that the amplified signal matches the range ofthe analog-to-digital converter and thus increases resolution. In someembodiments, the system may account for the gain using hardware,software, or any combination thereof, such that the original intensityinformation is retained.

In step 906, the system may determine a physiological parameter usinginformation from the attenuated photonic signals. The physiologicalparameter may be determined using any suitable hardware technique,software technique, or combination thereof. In some embodiments,processing equipment remote to the system may be used to determinephysiological parameters. The system may display the determinedphysiological parameter using a local display (e.g., display 320 of FIG.3 or display 328 of FIG. 3), display them on a remote display, publishthe data to a server or website, make the parameters available to a userby any other suitable technique, or any combination thereof.

FIG. 10 shows another illustrative timing diagram 1000 of aphysiological monitoring system in accordance with some embodiments ofthe present disclosure. In some embodiments, the system may vary a lightdrive signal such that a parameter is varied concurrent with diastoleperiods of the cardiac cycle. Timing diagram 1000 may include timeperiods related to the cardiac cycle, including systole period 1002,diastole period 1004, systole period 1006, and diastole period 1008.Timing diagram 1000 may include light drive signal 1010, shown in “on”and “off” states without units on the ordinate axis. It will beunderstood that light drive signal 1010 may provide a light drive signalto one or more emitters. It will also be understood any suitable numberof cardiac cycle modulation techniques may be used with any suitablenumber of emitters.

In some embodiments, the system may modulate light drive signal 1010 ina way related to the cardiac cycle. For example, light drive signal 1010may correspond to the light drive signal generated at step 902 of FIG.9. In some embodiments, the system may turn off a light drive signal forsystole period 1002, turn on the light drive signal for diastole period1004, turn off the light drive signal for systole period 1006, and turnon the light drive signal for diastole period 1008. Thus, the cardiaccycle modulation applied to light drive signal 1010 may be substantiallysynchronous with the diastole periods of the cardiac cycle. In someembodiments, the system may turn on a light source before diastoleperiods 1004 and 1008 and turn off a light source following diastoleperiods 1004 and 1008 so that the photonic signal can stabilize, so thatthe detector can stabilize, so that the processing equipment can obtainextra samples for averaging, interpolating, or decimating, for amplifiergain adjustments to stabilize, for any other suitable reason, or anycombination thereof.

The system may determine the time offsets between the “on” periods andthe region of interest based on user input, predetermined parameters,previous measurements, other suitable parameters, or any combinationthereof. The system may determine the timing of the diastole periodsusing historical information from previous cardiac cycles, from anexternal sensor, from user input, from measurements made with adifferent cardiac cycle modulation, by any other suitable technique, orany combination thereof. For example, the system may desire to align aparticular portion of the received signal with the center of amodulation cycle (or other suitable criteria) and advance or delaysubsequent modulation cycles to improve the alignment. It will beunderstood that the system may align modulation with pulses of theheart, pulses of a particular muscle group, other suitable pulses, anyother suitable physiological function, or any combination thereof.

It will also be understood that modulation of the light drive signal(i.e., the “on” and “off” states illustrated by light drive signal 1010)is merely exemplary and may include modulation of parameters includingdrive current or light brightness, duty cycle, firing rate, modulationparameters, other suitable parameters, or any combination thereof. Itwill also be understood that the “on” and “off” states are merelyexemplary and that the system may use any suitable variations ofdiscrete and/or continuous modulations. For example, discretemodulations may include drive signals with one or more step functions.Continuous modulations may include sinusoidal waveforms.

FIG. 11 shows another illustrative timing diagram 1100 of aphysiological monitoring system in accordance with some embodiments ofthe present disclosure. In some embodiments, the system may vary a lightdrive signal such that a parameter is increased concurrent with systoleperiods of the cardiac cycle. Timing diagram 1100 may include timeperiods related to the cardiac cycle, including systole period 1102,diastole period 1104, systole period 1106, and diastole period 1108.Timing diagram 1100 may include light drive signal 1110, shown in “on”and “off” states without units on the ordinate axis. It will beunderstood that light drive signal 1110 may provide a light drive signalto one or more emitters. It will also be understood any suitable numberof cardiac cycle modulation techniques may be used with any suitablenumber of emitters.

In some embodiments, the system may modulate light drive signal 1110 ina way related to the cardiac cycle. Light drive signal 1110 maycorrespond to the light drive signal generated at step 902 of FIG. 9. Insome embodiments, the system may turn on a light drive signal forsystole period 1102, turn off the light drive signal for diastole period1104, turn on the light drive signal for systole period 1106, and turnoff the light drive signal for diastole period 1108. Thus, the cardiaccycle modulation applied to light drive signal 1110 may be substantiallysynchronous with the systole periods of the cardiac cycle. In someembodiments, the system may turn on a light source before systoleperiods 1102 and 1106 and turn off a light source following systoleperiods 1102 and 1106 so that the photonic signal can stabilize, so thatthe detector can stabilize, so that the processing equipment can obtainextra samples for averaging, interpolating, or decimating, for amplifiergain adjustments to stabilize, for any other suitable reason, or anycombination thereof. The system may determine the time offsets betweenthe “on” periods and the region of interest based on user input,predetermined parameters, previous measurements, other suitableparameters, or any combination thereof. The system may determine thetiming of the systole periods using historical information from previouscardiac cycles, from an external sensor, from user input, frommeasurements made with a different cardiac cycle modulation, by anyother suitable technique, or any combination thereof. It will beunderstood that the system may align modulation with pulses of theheart, pulses of a particular muscle group, other suitable pulses, anyother suitable physiological function, or any combination thereof. Itwill also be understood that modulation of the light drive signal (i.e.,the “on” and “off” states illustrated by light drive signal 1110) ismerely exemplary and may include modulation of parameters includingdrive current or light brightness, duty cycle, firing rate, modulationparameters, other suitable parameters, or any combination thereof. Itwill also be understood that the “on” and “off” states are merelyexemplary and that the system may use any suitable number of discrete orcontinuous modulation variations.

FIG. 12 shows another illustrative timing diagram 1200 of aphysiological monitoring system in accordance with some embodiments ofthe present disclosure. In some embodiments, the system may vary a lightdrive signal such that a parameter is varied concurrent with peaks ortroughs in the PPG signal. Timing diagram 1200 may include PPG signal1202 and light drive signal 1204. PPG signal 1202 is shown witharbitrary units on the ordinate axis. Light drive signal 1204 is shownin “on” or “off” states without units associated with the ordinate axis.It will be understood that light drive signal 1204 may provide a lightdrive signal to one or more emitters. It will also be understood anysuitable number of cardiac cycle modulation techniques may be used withany suitable number of emitters.

In some embodiments, the system may modulate light drive signal 1202 ina way related to the cardiac cycle. Light drive signal 1204 maycorrespond to the light drive signal generated at step 902 of FIG. 9. Insome embodiments, the system may turn on a light drive signal for point1206 and point 1208. In some embodiments, point 1206 and 1208 mayrepresent peaks or troughs, depending on the orientation of the PPGsignal. Thus, the cardiac cycle modulation applied to light drive signal1204 may be substantially synchronous with the troughs of the PPGsignal. In some embodiments, the PPG signal may be inverted such thatthe periods of interest as illustrated in plot 1200 are peaks. It willbe understood that the system may identify periods of interest at anypoint in the cardiac cycle and that those illustrated in plot 1200 aremerely exemplary. For example, periods of interest may include peaks,valleys, troughs, dicrotic notches, fiducial points, other suitablepoints, or any combination thereof. In some embodiments, the light drivesignal may be in an “on” mode for both the peak and trough of a PPGsignal, and in an “off” mode for the rising and falling portion of thePPG signal. In some embodiments, the system may turn on a light sourcebefore peaks 1206 and 1208 and turn off a light source following peaks1206 and 1208 so that the photonic signal can stabilize, so that thedetector can stabilize, so that the processing equipment can obtainextra samples for averaging, interpolating, or decimating, for amplifiergain adjustments to stabilize, for any other suitable reason, or anycombination thereof. The system may determine the time offsets betweenthe “on” periods and the region of interest based on user input,predetermined parameters, previous measurements, other suitableparameters, or any combination thereof. The system may determine thetiming of the PPG peaks using information from previous cardiac cycles,from an external sensor, from user input, from measurements made with adifferent cardiac cycle modulation, by any other suitable technique, orany combination thereof. It will be understood that the system may alignmodulation with pulses of the heart, pulses of a particular musclegroup, other suitable pulses, any other suitable physiological function,or any combination thereof. It will also be understood that modulationof the light drive signal (i.e., the “on” and “off” states illustratedby light drive signal 1204) is merely exemplary and may includemodulation of parameters including drive current or light brightness,duty cycle, firing rate, modulation parameters, other suitableparameters, or any combination thereof. It will also be understood thatthe “on” and “off” states are merely exemplary and that the system mayuse any suitable number of discrete or continuous modulation variations.

FIG. 13 shows another illustrative timing diagram 1300 of aphysiological monitoring system in accordance with some embodiments ofthe present disclosure. In some embodiments, the system may vary a lightdrive signal such that a parameter is varied concurrent with receivingan external trigger, for example, an ECG. Timing diagram 1300 mayinclude time periods related to the cardiac cycle, including systoleperiod 1302, diastole period 1304, systole period 1306, and diastoleperiod 1308. Timing diagram 1300 may include ECG signal 1310,ECG-triggered pulse signal 1312, and light drive signal 1314. ECG signal1310 and ECG-triggered pulse signal 1312 are shown with arbitrary unitson the ordinate axis. Light drive signal 1314 is shown in “on” or “off”states without units associated with the ordinate axis. It will beunderstood that light drive signal 1314 may provide a light drive signalto one or more emitters. It will also be understood any suitable numberof cardiac cycle modulation techniques may be used with any suitablenumber of emitters.

In some embodiments, the system may modulate light drive signal 1314 ina way related to the cardiac cycle. Light drive signal 1314 maycorrespond to the light drive signal generated at step 902 of FIG. 9. Insome embodiments, the system may receive ECG signal 1310 from an ECGsensor, from an external system, from any other suitable source, or anycombination thereof. The system may process the ECG signal to determinea point of interest in the ECG signal. For example, the system mayidentify R wave peaks 1316 and 1318 in ECG signal 1310. In someembodiments, the systole period may begin at a time substantiallycorrelated to the R wave peak. It will be understood that any suitablefeature of any suitable external signal may be used to trigger changesin light drive signal modulation. For example, other features of the ECGmay be identified. In a further example, a time offset from a particularECG signal feature may correlate the cardiac cycle modulation with adesired cardiac cycle feature (e.g., a particular number of millisecondsin advance or delay of a particular ECG feature). In a further example,the system may identify features of other external signals such as anEEG, respiration rate, any other suitable signal, or any combinationthereof.

The system may generate ECG-triggered pulse signal 1312 including asignal pulse 1320 substantially concurrent with R wave peak 1316 andsignal pulse 1322 substantially concurrent with R wave peak 1318. Insome embodiments, the system may determine the timing of theECG-triggered pulse cycles using historical information from previouscardiac cycles in addition to the instant pulse cycle. In someembodiments, the system may receive only ECG-triggered pulse signal 1312(i.e., the ECG-triggered pulse signal may be generated by an externalsystem). In some embodiments, the system may generate ECG-triggeredpulse signal 1312. In some embodiments, the system may receiveECG-triggered pulse signal 1312 from user input or from an externalprocessing device not related to the ECG. In some embodiments, thesystem may modulate light drive signal 1314 in a way correlated toECG-triggered pulse signal 1312, ECG signal 1310, any other suitablesignal, or any combination thereof. For example, the system may changelight drive signal 1314 during period 1324 to an “on” state in responseto signal pulse 1320 and during period 1326 to an “on” state in responseto signal pulse 1322. In some embodiments, the system may delay oradvance periods 1324 and 1326 in relation to signal pulses 1320 and1322, respectively. For example, the system may delay or advance the“on” state of the light drive signal to account for a delay between theECG signal measured near the heart and the features of a PPG signalmeasured by a sensor remote from the heart (e.g., on a digit). In someembodiments, multiple signal pulses or multiple pulse signals (notshown) may be used to change the state of the light drive signal. Forexample, the system may receive a second signal pulse associated with aphysiological parameter to turn the light drive signal to an “off”state. In some embodiments, the duration of the “on” state following asignal pulse (e.g., signal pulse 1320) may be a predetermined length oftime, a length of time determined by previous measurements, a length oftime determined by data collected during previous periods, a length oftime set by user input, a length of time determined by any othersuitable technique, or any combination thereof.

It will be understood that the system may align light drive signalmodulation with pulses of the heart, pulses of a particular musclegroup, other suitable pulses, any other suitable physiological function,or any combination thereof. It will also be understood that modulationof the light drive signal (i.e., the “on” and “off” states illustratedby light drive signal 1314) is merely exemplary and may includemodulation of parameters including drive current or light brightness,duty cycle, firing rate, modulation parameters, other suitableparameters, or any combination thereof. It will also be understood thatthe “on” and “off” states are merely exemplary and that the system mayuse any suitable number of discrete or continuous modulation variations.

FIG. 14 shows another illustrative timing diagram 1400 of aphysiological monitoring system in accordance with some embodiments ofthe present disclosure. In some embodiments, the system may vary thelight drive signal using a technique that skips cardiac cycles. Timingdiagram 1400 may include time periods related to the cardiac cycle,including pulse cycles 1406, 1408, 1410, 1412, 1414, 1416, and 1418.Timing diagram 1400 may include PPG signal 1402, and light drive signal1404. PPG signal 1402 is shown with arbitrary units on the ordinateaxis. Light drive signal 1404 is shown in “on” or “off” states withoutunits associated with the ordinate axis. It will be understood thatlight drive signal 1404 may provide a light drive signal to one or moreemitters. It will also be understood any suitable number of cardiaccycle modulation techniques may be used with any suitable number ofemitters.

The system may vary the light drive signal in a way correlated to thecardiac pulse signal of the subject. The system may generate an “on”light drive signal during one or more cardiac cycles and an “off” lightdrive signal during one or more cardiac cycles. For example, the systemmay generate an “on” light drive signal during period 1420 concurrentwith pulse cycle 1406, an “off” light drive signal during pulse cycles1408 and 1410, an “on” light drive signal during period 1422 concurrentwith pulse cycle 1412, an “off” light drive signal during pulse cycles1414 and 1416, and an “on” light drive signal during period 1424concurrent with pulse cycle 1418. It will be understood that thisparticular modulation is merely exemplary and that any suitableinter-cycle modulation may be used.

In some embodiments, the system may use an intra-cardiac cyclemodulation as described above during “on” periods 1420, 1422, and 1424of the inter-cardiac cycle modulation illustrated in timing diagram1400. For example, the system may emit light during any suitableintra-cycle period (e.g., systole period, diastole period, peak, etc.)of pulse cycle 1406, 1412, and 1418, as described in flow diagram 900 ofFIG. 9. In some embodiments, the system may emit light from a firstlight source during the “off” cardiac cycles and emit light from asecond light source during the “on” periods (or a portion of the “on”periods), such that information related to the first light source isused to determine when to generate the second light drive signal asdescribed in flow diagram 400 of FIG. 4.

The system may determine the timing of the cardiac cycles usinghistorical information from previous cardiac cycles, from an externalsensor, from user input, from measurements made with a different cardiaccycle modulation, by any other suitable technique, or any combinationthereof. It will be understood that the system may align modulation withpulses of the heart, pulses of a particular muscle group, other suitablepulses, any other suitable physiological function, or any combinationthereof. It will also be understood that modulation of the light drivesignal (i.e., the “on” and “off” states illustrated by light drivesignal 1110) is merely exemplary and may include modulation ofparameters including drive current or light brightness, duty cycle,firing rate, modulation parameters, other suitable parameters, or anycombination thereof. It will also be understood that the “on” and “off”states are merely exemplary and that the system may use any suitablenumber of discrete or continuous modulation variations.

FIG. 15 shows another illustrative timing diagram 1500 of aphysiological monitoring system in accordance with some embodiments ofthe present disclosure. In some embodiments, the system may vary a lightdrive signal such that a parameter is varied continuously according to aperiodic waveform. Timing diagram 1500 may include time periods relatedto the cardiac cycle, including systole period 1502, diastole period1504, systole period 1506, and diastole period 1508. Timing diagram 1500may include PPG signal 1510 and light drive signal 1512, shown witharbitrary units on the ordinate axis. It will be understood that lightdrive signal 1512 may provide a light drive signal to one or moreemitters. It will also be understood any suitable number of cardiaccycle modulation techniques may be used with any suitable number ofemitters.

In some embodiments, the system may modulate light drive signal 1512 ina way related to the cardiac cycle. Light drive signal 1512 maycorrespond to the light drive signal generated at step 902 of FIG. 9. Insome embodiments, the system may apply a waveform modulation to lightdrive signal 1512 such that the waveform maxima are substantiallyaligned with a cardiac cycle feature. For example, the system may alignpeak 1520 of light drive signal 1512 with systole period 1502. Theamplitude of light drive signal 1512 may relate to drive current orlight brightness, duty cycle, firing rate, modulation parameters, othersuitable parameters, or any combination thereof. In some embodiments,the system may superimpose the amplitude of light drive signal 1512 as acardiac cycle modulation envelope function on the amplitudes of a drivecycle modulation.

In some embodiments, the system may determine the position of systoleperiod 1502 using PPG signal 1510. For example, the system may apply atime offset to PPG peak 1516 or PPG trough 1518 to determine the centerof the systole period, and the system may adjust the light drive signalsuch that peak 1520 is aligned with the center of the systole period. Itwill be understood that the system may align any suitable feature of thewaveform of light drive signal 1512 with any one or more suitablefeatures of the cardiac cycle. For example, the waveform may be alignedsuch that peak 1520 is correlated with diastole period 1504. In someembodiments, the waveform may be aligned with features of the PPGsignal. For example, peak 1520 may be aligned with peak 1516, trough1518, or a dicrotic notch (not shown). It will be understood that thesealignments are merely exemplary and that any suitable feature of onesignal may be aligned with any suitable feature of the other.

In some embodiments, the system may modulate light drive signal 1512with any suitable periodic waveform, for example, a square wave,triangle wave, sawtooth wave, sinusoidal wave, any other suitable wave,or any combination thereof. In some embodiments, continuous periodicwaveforms may be preferable to discontinuous waveform (e.g., squarewaves) with regards to further processing steps. In some embodiments,the system may determine the alignment of the waveform using informationfrom previous cardiac cycles, from an external sensor, from user input,from measurements made with a different cardiac cycle modulation, by anyother suitable technique, or any combination thereof. It will beunderstood that the system may align modulation with pulses of theheart, pulses of a particular muscle group, other suitable pulses, anyother suitable physiological function, or any combination thereof. Itwill be understood that the system may implement cardiac cyclemodulations in addition to drive cycle modulations and conventionalservo algorithms. It will be understood that the amplitudes of lightdrive signal 1512 may represent amplitudes of any suitable cardiac cyclemodulation parameter, as described above. For example, amplitudes oflight drive signal 1512 may relate to parameters including drive currentor light brightness, duty cycle, firing rate, modulation parameters,other suitable parameters, or any combination thereof.

FIG. 16 shows another illustrative timing diagram 1600 of aphysiological monitoring system in accordance with some embodiments ofthe present disclosure. In some embodiments, the system may vary a lightdrive signal such that a parameter is varied continuously according to aperiodic waveform. Timing diagram 1600 may include PPG signal 1610 andlight drive signal 1612, shown with arbitrary units on the ordinateaxis. It will be understood that light drive signal 1612 may provide alight drive signal to one or more emitters. It will also be understoodany suitable number of cardiac cycle modulation techniques may be usedwith any suitable number of emitters.

In some embodiments, the system may modulate light drive signal 1612 ina way related to the cardiac cycle. Light drive signal 1612 maycorrespond to the light drive signal generated at step 902 of FIG. 9. Insome embodiments, the system may apply a waveform modulation to lightdrive signal 1612 such that the waveform maxima are substantiallyaligned with a cardiac cycle feature. For example, the system may alignpeak 1620 of light drive signal 1612 with peak 1616 in PPG signal 1610.In another embodiment, the system may align peak 1620 of light drivesignal 1612 with PPG trough 1618, a dicrotic notch, a fiducial point,any other suitable feature, or any combination thereof. It will beunderstood that these alignments are merely exemplary and that anysuitable feature of one signal may be aligned with any suitable featureof the other. For example, the system may apply a time offset to PPGpeak 1616 or PPG trough 1618 to determine the desired location of peak1620.

The system may determine the timing of the cardiac cycles usinghistorical information from previous cardiac cycles, from an externalsensor, from user input, from measurements made with a different cardiaccycle modulation, by any other suitable technique, or any combinationthereof. It will be understood that the system may align modulation withpulses of the heart, pulses of a particular muscle group, other suitablepulses, any other suitable physiological function, or any combinationthereof. It will also be understood that modulation of the light drivesignal (i.e., the “on” and “off” states illustrated by light drivesignal 1612) is merely exemplary and may include modulation ofparameters including drive current or light brightness, duty cycle,firing rate, modulation parameters, other suitable parameters, or anycombination thereof. It will also be understood that the “on” and “off”states are merely exemplary and that the system may use any suitablenumber of discrete or continuous modulation variations.

FIG. 17 is flow diagram 1700 showing illustrative steps for decimatingand interpolating a signal in accordance with some embodiments of thepresent disclosure. In some embodiments, the system may sample a signalat different rates throughout a cardiac cycle. The system may processthe sampled signal to produce an output signal with a constant rate. Insome embodiments, the system may vary the sampling rate to reduce oroptimize power consumption. In some embodiments, sampling ratemodulation may be correlated with light drive signal modulation. Varyingthe sampling rate may reduce power consumption by reducing emitter drivetime and lowering utilization of an analog-to-digital converter. In someembodiments, varying the sampling rate may increase the time resolutionof identified features. For example, in a continuous non-invasive bloodpressure measurement where the pulse transit time is used incalculations, increasing the sampling rate for a portion of the cardiaccycle may result in more accurate and reliable physiologicalinformation. In another example, varying the sampling rate around acardiac pulse cycle feature, such as a peak or notch, may increase theaccuracy of determining the location of that feature in time. In someembodiments, lower frequency or less critical parts of the cardiac pulsecycle may be sampled at a lower rate while maintaining highly accuracyand reliable physiological parameter determination.

In step 1702, the system may receive a signal. The signal may be sampledat a first rate. The signal may be sampled using an analog-to-digitalconverter in the front end processing circuitry. For example, anattenuated light signal may be converted to an analog electronic signalby a detector. The analog electronic signal may be converted to adigital signal at a particular rate by an analog to digital converter,such that at each sampling point the converter determines the amplitudeof the analog signal (e.g., current or voltage) and outputs a digitalword related to that amplitude. The rate at which sampling occurs may becontrolled by, for example, a timing control signal (e.g., provided bycontrol circuitry 110 of FIG. 1). In some embodiments, the sampling ratemay represent the number of samples taken during an “on” period of thedrive cycle modulation. In some embodiments, the sampling rate mayrepresent the amount of time between “on” periods.

In step 1704, the system may receive a signal and sample it at a secondsampling rate. For example, the period between samples may be increasedor reduced. In some embodiments, this may relate to a higher resolutiondigitization of an analog signal. In some embodiments, changes in thesampling period may vary continuously according to some predeterminedperiodic waveform, may change as a step functions, may be varied by anyother suitable technique, or any combination thereof. In someembodiments, the system may store, communicate, or otherwise utilize thesampling rate along with the digitized sample. The sampling rate may beused in subsequent processing steps.

In step 1706, the system may decimate or interpolate the receivedsignals to output signals at a constant rate. In some embodiments, itmay be desirable for further processing to make the sampling rateconstant throughout a signal. In some embodiments, the system may selectthe first and second sampling rates to ease or improve the interpolationand decimation of step 1706. For example, the second rate may be aninteger multiple of the first rate.

The system may increase the sampling rate at a particular region in asignal by interpolation. Interpolation may add additional samples usinginformation from the existing samples. For example, a data point may beadded between two existing data points by calculating a mathematicalaverage of the existing data. The system may use more complexinterpolation schemes such as linear interpolation, polynomialinterpolation, spline interpolation, any other suitable interpolationscheme, or any combination thereof. Interpolation may includeupsampling, where zero valued segments are inserted into the digitalsignal. Upsampling may be followed by filtering to smooth the output. Insome embodiments, filtering may be carried out by processing equipmentdiscrete from sampling processing equipment, by integrated processingequipment, or any combination thereof. Interpolation may be carried outusing hardware equipment, software equipment, or any combinationthereof.

The system may decrease the sampling rate at a particular region in asignal by decimation. Decimation may decrease the sampling rate byremoving samples from a signal. Decimation techniques may includedownsampling, where segments of a sample are removed. Downsampling mayinclude or be followed by filtering to smooth the output signal. It willbe understood that a decimated signal originally sampled at a highersampling rate may provide improved data as compared to a signalinitially sampled at a lower rate. In some embodiments, interpolationmay be used to decrease the sampling rate. For example, interpolationmay decrease the sampling rate by replacing samples in a signal with asmaller number of samples.

The desired sampling rate may be determined based on the highestsampling rate available in the samples, the lowest sampling rateavailable in the samples, the downstream processing equipment, any othersuitable determining factors, or any combination thereof. In someembodiments, a signal may be decimated, interpolated, or both, dependingon the sampling rates used throughout and the desired output samplingrate. It will be understood that the output rate may not be a constantrate. It will also be understood that sampling rate is one of thecomponents that may be modulated in cardiac cycle modulation asdescribed above. It will also be understood that the earlier describedembodiments relating to varying light output may also apply to samplingrate.

In some embodiments, the sampling rate may represent the number ofsamples taken during an “on” period of the drive cycle modulation. Forexample, an “on” period may be “on” period 202 of FIG. 2A. The systemmay sample that drive cycle modulation “on” period a lower number oftimes at a low sampling rate, and a greater number of times at a highsampling rate. In this embodiment, the firing rate of the emitters maynot be modulated along with the analog-to-digital sampling rate. Forexample, at a low sampling rate, the system may sample once per “on”period, while at a high sampling rate the system may take severalsamples per “on” period and average them. In some embodiments, averagingmultiple samples for the same “on” period may include oversamplingtechniques.

In some embodiments, the sampling rate may represent the amount of timebetween “on” periods. For example, the time between “on” periods may bethe length of time of “off” period 220 of FIG. 2A. Increasing theduration of the “off” periods (i.e., decreasing the emitter firing rate)relates to a decreased sampling rate. Similarly, decreasing the durationof the “off” periods (i.e., increasing the emitter firing rate) relatesto an increased sampling rate.

FIG. 18 shows an illustrative timing diagram of a physiologicalmonitoring system including sampling rate variation in accordance withsome embodiments of the present disclosure. Plot 1800 may includesystole periods 1802 and 1806, and diastole periods 1804 and 1808. Plot1800 may include PPG signal 1810, light drive signal 1812, raw samplingrate signal 1814, and decimated sampling rate signal 1816.

The vertical lines illustrated in raw sampling rate signal 1814 anddecimated sampling rate signal 1816 may be indicative of individualsamples, or a representative sampling rate, as described above. Thus, anincreased number of vertical lines in a region may be indicative of arelatively higher sampling rate, while a decreased number of verticallines in a region may be indicative of a relatively lower sampling rate.The system may vary other cardiac cycle modulation parameters along withsampling rate. For example, as illustrated by light drive signal 1812,the system may sample at a high rate during high light drive signalperiod 1820, as indicated by high raw sampling rate period 1822 andsample at a low rate during lower during a low light output drive signal1830, as indicated by low raw sampling rate period 1826. The amplitudeof light drive signal 1812 may relate to modulation of parametersincluding drive current or light brightness, duty cycle, firing rate,modulation parameters, other suitable parameters, or any combinationthereof. It will be understood that the square wave modulation of lightdrive signal 1812 is merely exemplary and that any suitable modulationtechnique may be used.

In the example illustrated in plot 1800, the high light drive signalperiod 1820 is substantially aligned with systole period 1802, though itwill be understood that the system may use any suitable cardiac cyclemodulation technique and may correlate modulation features with anysuitable signal elements. The system may use a relatively high rawsampling rate period 1822 for a period substantially aligned with highlight drive signal period 1820. The system may use a relatively low rawsampling rate period 1826 for a period substantially aligned with lowlight drive signal period 1830.

In some embodiments, the system may use a high sampling rate during thehigh light output periods and a low sampling rate during low lightoutput periods to reduce power consumption while still obtaining highquality determinations of physiological parameters. It will beunderstood that the particular sampling rates shown in plot 1800 aremerely exemplary and that the system may employ any suitable samplingrate and any suitable sampling rate modulation.

In some embodiments, the system may process raw sampling rate signal1814 to produce decimated sampling rate signal 1816 by decimation,interpolation, any other suitable sampling rate modification, or anycombination thereof. As used herein, “raw” refers to the sampling rateat the analog-to-digital converter prior to interpolation or decimation.It will be understood that the system may decimate and interpolatesamples using any suitable hardware technique, software technique, orany combination thereof.

In the example illustrated in plot 1800, the raw sampling rate signal1814 during high raw sampling rate period 1822 is decimated to a lowersampling rate in one or more processing steps to produce decimatedsampling rate signal 1816 in sampling rate period 1824. Low samplingrate period 1826 of raw sampling rate signal 1814 is kept the same insampling rate period 1828 of decimated sampling rate signal 1816. Thus,while the sampling rate in raw sampling rate signal 1814 variesthroughout the cardiac cycle, the sampling rate in the decimatedsampling rate signal 1816 is constant throughout the cardiac cycle. Inanother embodiment (not shown), the sampling during low sampling rateperiod 1826 may be interpolated to increase the sampling rate to matchthat of high sampling rate period 1822 to produce a decimated samplingrate signal 1816 with a constant sampling rate. In another embodiment,sampling rates may be decimated, interpolated, or a combination thereofto achieve a constant sampling rate in decimated sampling rate signal1816.

In some embodiments, the system may alter sampling rate modulation,sampling rate decimation, and sampling rate interpolation to optimizepower consumption. For example, the system may use a modulated methodwhen powered by battery power and a constant sampling rate when poweredby an external power source. In some embodiments, the sampling rate maybe modulated using a step function (as illustrated in FIG. 18), bymultiple step functions, by a periodic continuous function, by any othersuitable modulation, or any combination thereof.

FIG. 19 is flow chart 1900 showing steps to adjust a cardiac cyclemodulation based on a physiological condition in accordance with someembodiments of the present disclosure.

In step 1902, the system may perform a physiological measurement in afirst mode. The first mode may be, for example, a first cardiac cyclemodulation technique.

In step 1904, the system may detect a physiological condition. Forexample, the system may detect dysrhythmia, arrhythmia, fibrillation,non-periodic heartbeat, tachycardia, other cardiac irregularity, or anycombination thereof. The system may also detect no heartbeat. In someembodiments, the system may false-positive detect a physiologicalcondition in the presence of noise, ambient light, a loss of detectorsignal, any other suitable reason, or any combination thereof. In someembodiments, the system may detect a physiological parameter, forexample, a low blood oxygen saturation, low pulse rate, high pulse rate,high blood pressure, low blood pressure, any other suitable condition,or any combination thereof. In some embodiments, the system may detect asignal indicative of a system error such as a physiologically impossiblevalue, a probe-off signal, any other suitable signal, or any combinationthereof. The system may detect the physiological condition usinginformation obtained through measurements in step 1902, from an externalsensor or controller, by any other suitable technique, or anycombination thereof.

In some embodiments, the system may detect a condition where a secondmode is required that is not related to a physiological condition. Forexample, the system may detect a change in background noise, a change inambient light, a change in the available power, other suitable changes,or any combination thereof. In some embodiments, an increase in thenumber of identified fiducial points (e.g., zero crossings of thederivatives) may cause the system to vary drive signals.

In step 1906, the system may perform a physiological measurement in asecond mode. For example, the system may stop cardiac cycle modulationand emit light at a constant brightness. In a further example, thesystem may increase the emitter intensity used in a cardiac cyclemodulation. In a further example, the system may lengthen the “on”periods of a cardiac cycle modulation. In a further example, the systemmay alter the cardiac cycle modulation technique as described above withrelation to FIG. 8B. In some embodiments, a non-periodic heartbeat maymake cardiac cycle modulation not desirable or not possible. In someembodiments, the system may return to the first mode after the detectedphysiological condition ceases, after a predetermined time period, afterreceiving user input, after any other suitable command, or anycombination thereof. In some embodiments, when performing themeasurement in a second mode, the system may also notify the user by analarm, by a display, by any other suitable notification technique, orany combination thereof.

FIG. 20 is illustrative timing diagram 2000 of a system operating in afirst and second mode following detection of a physiological conditionin accordance with some embodiments of the present disclosure. Plot 2000may include PPG signal 2004 and cardiac cycle modulation 2006. PPGsignal 2004 may include normal cardiac waveform period 2008 and abnormalcardiac waveform period 2010. For example, abnormal cardiac waveformperiod 2010 may include the heart ceasing to beat (i.e., “flatline”) orentering a cardiac dysrhythmia (e.g., ventricular fibrillation, atrialfibrillation, AV blockage). At point 2018, the system may detect in PPGsignal 2004 the beginning of abnormal cardiac waveform period 2010. Asdescribed above, the system may detect abnormal cardiac waveform period2010 by processing PPG signal 2004 during normal cardiac internal 2008,by an external trigger, from any other suitable input, or anycombination thereof. In some embodiments, the system may output lightusing cardiac cycle modulation 2006. For example, the system may outputlight or vary any suitable light drive signal component with sinusoidalmodulation during period 2012 substantially aligned with normal cardiacoperation period 2008. The system may change the cardiac cyclemodulation technique at point 2016 based on the abnormal cardiacwaveform identified at point 2018. For example, the system may outputlight at constant brightness during period 2014 substantially alignedwith abnormal cardiac waveform period 2010. It will be understood thatsinusoidal modulation 2012 is merely exemplary and that the system mayuse any suitable modulation technique. It will also be understood thatthere may be a time offset between identifying the abnormal cardiacwaveform at point 2018 and modifying the modulation technique at point2016, and that the delay illustrated in plot 2000 is merely exemplary.

FIG. 21 is another illustrative timing diagram 2100 of a systemoperating in a first and second mode following detection of aphysiological condition in accordance with some embodiments of thepresent disclosure. Plot 2100 may include PPG signal 2102, red lightdrive signal 2104 and IR light drive signal 2016. PPG signal 2102 mayinclude normal cardiac waveform period 2108 and abnormal cardiacwaveform period 2110. For example, abnormal cardiac waveform period 2110may include the heart ceasing to beat or entering a cardiac dysrhythmia.At point 2112, the system may detect in PPG signal 2102 the start ofabnormal cardiac waveform period 2110. As described above, the systemmay detect abnormal cardiac waveform period 2110 by processing PPGsignal 2102 during normal cardiac waveform interval 2108, by an externaltrigger, from any other suitable input, or any combination thereof. Insome embodiments, the system may output light using a cardiac cyclemodulation shown by red light drive signal 2104 and IR light drivesignal 2016. For example, the system may output IR light with a constantoutput, as indicated by the solid block of IR light drive signal 2016.The system may modulate red light output with a square wave, asindicated by the broken blocks of red light drive signal 2104 inmodulated light period 2114. The system may change the cardiac cyclemodulation technique at point 2118 based on the abnormal cardiacwaveform identified at point 2112 in PPG 2102. For example, the systemmay output light at constant brightness during period 2116 substantiallyaligned with abnormal cardiac waveform period 2110. In a furtherexample, where in the first mode the system emits light from a firstlight source at a constant rate and a modulated intensity from a secondlight source, the system may switch to a constant brightness for boththe first and second light sources. It will be understood that the swinewave modulation of modulated light period 2114 is merely exemplary andthat the system may use any suitable modulation technique. It will alsobe understood that there may be a time delay between identifying theabnormal cardiac operation at point 2112 and modifying the modulationtechnique at point 2118, and that the delay illustrated in plot 2100 ismerely exemplary.

FIG. 22 is flow diagram 2200 showing illustrative steps for identifyingfeatures in a signal in accordance with some embodiments of the presentdisclosure. In some embodiments, the system may perform processing stepsto identify points of interest in a signal. The signal may be, forexample, a received attenuated photonic signal. In some embodiments, thesystem may use the identified points of interest in adjusting cardiaccycle modulation. Identified points of interest may include local maximaand minima of a PPG signal, fiducial points, any other suitable points,or any combination thereof. In some embodiments, identification offiducials may be intra-channel, where information from a signal is usedto identify fiducials in that signal. In some embodiments,identification of fiducial may be inter-channel, where information froma first signal is used to identify fiducials in a second signal. Thesystem may use any suitable combination of inter-channel andintra-channel identification techniques. The selection of aninter-channel or intra-channel identification technique may depend, inpart, on the light sources and the cardiac cycle modulation technique.

In step 2202, the system may receive a signal. The signal may be, forexample, an attenuated photonic signal. The signal may have beendetected by a detector. The detected signal may be processed byprocessing equipment including, for example, digitizers, filters,decimators, interpolators, other suitable processing equipment, or anycombination thereof. In some embodiments, the system may amplify thereceived signal using front end processor circuitry. The gain of theamplifier may be adjusted based on the emitted light brightness,historical information related to the brightness of prior receivedattenuated signals, other suitable information, or any combinationthereof, so that the amplified signal matches the range of theanalog-to-digital converter and thus increases resolution. In someembodiments, the system may account for the gain using hardware,software, or any combination thereof, such that the original intensityinformation is retained.

In step 2204, the system may calculate a second signal related to thefirst signal. For example, the system may calculate the derivative ofthe signal. In some embodiments, the system may calculate the second,third, fourth, or any other suitable derivative of the signal. The“derivative” is understood to be the rate of change of a signal, andn-th (where n is understood to represent a positive integer) derivativesare understood to be iterative applications of the derivativecalculation. In some embodiments, the system may calculate an integral,moving average, any other suitable function, or any combination thereof.“Fiducials” are understood to represent points of interest that maycorrespond to features in a signal. Fiducials may be associated withzero crossings of the derivative of a signal.

In step 2206, the system may identify features of the second signal suchas fiducials. In some embodiments, the system may identify zerocrossings of the second signal. In some embodiments, the system mayidentify crossings of a non-zero threshold.

In step 2208, the system may correlate identified features of the secondsignal with the first signal. In some embodiments, when the secondsignal is the first derivative of the first signal, the system maycorrelate zero crossings of the second signal as local maxima and minimaof the first signal. In some embodiments, where the second signal is thesecond derivative of the first signal, the system may correlate zerocrossings of the second signal as inflection points of the first signal.The system may use higher order derivatives to identify other points inthe first signal. In some embodiments, zero crossings of the secondderivative may be associated with cardiovascular aging.

In some embodiments, a non-zero threshold crossing of the second signalmay be used in determining cardiac cycle modulation (illustrated in FIG.25 discussed below). For example, where the system desires to measure afeature located at the zero crossing of the second signal, it may turnon the light source when the signal is at some small, non-zero levelsuch that the light source will be stabilized once the zero crossing isreached. Similarly, the system may use a subsequent non-zero crossing asa turn-off point.

In some embodiments, the system may use a photonic signal from a firstlight source to determine fiducials and other points of interest, andmodulate a second light source based on the information determined usingthe first light source (e.g., using the method described by flow diagram400 of FIG. 4). In some embodiments, the system may use fiducials andother points to modulate light drive signals, sampling rates, othersuitable parameters, or any combination thereof.

It will be understood that the above identified algorithms foridentifying fiducials and other points of interest are merely exemplaryand that the system may use any suitable algorithm or technique,implemented in hardware, software, or any combination thereof.

FIG. 23 is illustrative plot 2300 of a waveform showing identificationof fiducials in accordance with some embodiments of the presentdisclosure. Plot 2300 may include waveform 2304 and first derivative2302, where first derivative 2302 is determined by calculating thederivative of waveform 2304. Waveform 2304 may be the first signal ofstep 2202 of FIG. 22 and first derivative 2302 may be the second signalof step 2204. In some embodiments, zero crossings of the firstderivative may be identified as fiducial points. Zero crossing 2306 offirst derivative 2302 may be identified, for example, in step 2206 ofFIG. 22. The system may identify local minimum 2308 of waveform 2302 inrelation to zero crossing 2306. In some embodiments, the system mayidentify the negative-to-positive nature of zero crossing 2306 and thusidentify local minimum 2308 as a minimum. Similarly, the system mayidentify the positive-to-negative nature of zero crossing 2310 and thusidentify local maximum 2312 as a maximum.

FIG. 24 is another illustrative plot 2400 of a waveform showingidentification of fiducials in accordance with some embodiments of thepresent disclosure. In some embodiments, the system may identify thezero crossings of a second derivative and relate them to fiducial pointson the received signal.

Plot 2400 may include waveform 2402, first derivative 2404 and secondderivative 2406, where the first and second derivatives are derivativesof waveform 2402, as described above. In some embodiments, the systemmay identify the zero crossings of second derivative 2406. For example,second derivative 2406 may have a zero crossing at point 2410. Thesystem may relate the position of point 2410 with point 2412 on waveform2402. In some embodiments, the system may determine the amplitude ofwaveform 2402 at point 2412 and include that information in furtherprocessing of a physiological parameter. For example, informationrelated to the zero crossings of second derivatives may be used in partin determining cardiovascular aging. In some embodiments, informationfrom multiple derivatives may be combined. For example, when the firstderivative is approaching zero, the second derivative may be used toestimate how quickly it will approach zero. In another example, whenfiducial points are based on the second derivative, the third derivativemay be used in part to determine the optical light drive signal.

Waveform 2402 may be the first signal of step 2202 of FIG. 22 and secondderivative 2404 may be the second signal of step 2204. The zerocrossings of the second derivative may be the features identified instep 2206.

FIG. 25 is another illustrative plot 2500 of a waveform showingidentification of fiducials in accordance with some embodiments of thepresent disclosure. In some embodiments, the system may use non-zerothreshold crossings to identify a region of interest surrounding afiducial or other point of interest. In some embodiments, the system mayturn on a light source before a desired point of interest and turn off alight source following a point of interest so that the photonic signalcan stabilize, so that the detector can stabilize, so that theprocessing equipment can obtain extra samples for averaging,interpolating, or decimating, for any other suitable reason, foramplifier gain adjustments to stabilize, or any combination thereof. Insome embodiments, the system may use historical information fromprevious pulse cycles in determining thresholds. In some embodiments,the system may collect data without using cardiac cycle modulation untilenough information is collected to determine thresholds and otheralignment information for cardiac cycle modulation. In some embodiments,the system may adjust thresholds using historical information fromprevious cardiac cycle modulated pulse cycles. In some embodiments, thesystem may determine ensemble averages of previous pulse cycles. In someembodiments, the system may use multiple thresholds depending on theperiod of interest, the light source, the cardiac cycle modulation, thedrive cycle modulation, convention servo algorithms, other suitablecriteria, or any combination thereof. In some embodiments, the systemmay use the shape, slope, trend, derivatives, other suitableinformation, and any combination thereof, to determine parameters forcardiac cycle modulation (e.g., emitter brightness).

Plot 2500 may include waveform 2502 and derivative 2504. Thresholds mayinclude zero threshold 2506, positive threshold 2508, and negativethreshold 2510. In some embodiments, the positive and negative thresholdoffsets from zero may or may not be equal. In some embodiments, theoffsets may be determined by user input, by predetermined values, byprocessing of previous data, by system settings related to the sensorand detector, by system settings related to the physiological parameterdetermined, by any other suitable parameters, or any combinationthereof.

The system may identify threshold crossings of derivative 2504. Forexample, the system may identify positive threshold crossing 2512, zerothreshold crossing 2516, and negative threshold crossing 2518. Thesystem may use these points to determine light drive signals, to varycardiac cycle modulation, to vary any other suitable parameters, or anycombination thereof. The system may correlate threshold crossings withpoints on waveform 2502, for example, correlating positive thresholdcrossing 2512 with point 2520, zero threshold crossing 2516 with point2422, and negative threshold crossing 2518 with point 2524. For example,in an embodiment described by flow diagram 400 of FIG. 4, where a secondphotonic signal is controlled using in part information determined by afirst photonic signal, positive threshold crossing 2512 may be used as aturn on point for the second photonic signal and negative thresholdcrossing 2518 may be used as a turn off point for the second photonicsignal. The system would thus sample waveform 2502 from point 2520 to2524 (i.e., segment 2526) with, for example, the second photonic signal.In some embodiments, information related to threshold crossings may beused to determine other information related to waveform 2502. Forexample, the order of positive and negative threshold crossings of afirst derivative may be used to identify a related zero crossing as alocal maximum or minimum.

FIG. 26 is illustrative plot 2600 of waveforms showing pulseidentification in accordance with some embodiments of the presentdisclosure. Plot 2600 may include PPG signal 2602, systole periodmodulated PPG signal 2604, and diastole period modulated PPG signal2606. Plot 2600 may be shown with arbitrary units on the ordinate axisand time on the abscissa. The signals shown in plot 2600 are simulatedsawtooth-shaped 60 BPM IR waveforms with a moderate amount of noise(e.g., Gaussian noise from approximately 0-5 Hz with amplitudeindependent of emitter output). The signals provide examples thatillustrate when cardiac cycle modulation is properly selected, theaccuracy of monitoring functions can be enhanced. The circles shown inplot 2600 indicate the occurrence of local minima and maxima using, forexample, a roughly 150 ms window centered on each sample.

PPG signal 2602 may be representative of a PPG signal deter mined usinga fixed power output throughout the cardiac cycle. Identified localmaximum 2608 and local minimum 2610 are representative of a correctlyidentified peak and valley of physiological pulse in PPG signal 2602.The identified local maximum and minimum in region 2611 arerepresentative of noise and do not correctly identify the peak andvalley of a physiological pulse.

Systole period modulated PPG signal 2604 may be representative of a PPGsignal determined using a cardiac cycle modulation, where the lightsource is modulated with a sinusoidal waveform that varies thebrightness of the emitter from 50-150% of the mean. The peak emitteroutput is centered during the systole period of the cardiac cycle. Forexample, the light drive signal may drive an IR modulated LED usingsinusoidal cardiac cycle modulation where peak LED output occurs duringsystole. In a further example, the cardiac cycle modulations may relateto cardiac cycle modulations illustrated in plot 500 of FIG. 5 and plot1100 of FIG. 11. Identified local maximum 2614 and local minimum 2612are representative of a correctly identified peak and valley of aphysiological pulse. The identified maxima and minima in, for example,regions 2616 and 2618 are representative of noise and do not correctlyidentify the peak and valley of a physiological pulse. Because systoleperiod modulated PPG signal 2604 uses lower power during the diastoleportion of the cardiac cycle, noise may have a greater influence andcause spurious local maxima and minima to appear in the diastole portionof signal 2604. In some embodiments, the increased effect of noiseduring the diastole portions may reduce the accuracy or reliability of apulse determination.

Diastole period modulated PPG signal 2606 may be representative of a PPGsignal determined using a cardiac cycle modulation, where the lightsource is modulated with a sinusoidal waveform that varies thebrightness of the emitter from 50-150% of the mean. The peak emitteroutput is centered during the diastole period of the cardiac cycle. Forexample, the light drive signal may drive an IR modulated LED usingsinusoidal cardiac cycle modulation where peak LED output occurs duringdiastole. In a further example, the cardiac cycle modulations may relateto cardiac cycle modulations illustrated in plot 600 of FIG. 6 and plot1000 of FIG. 10. Identified local maximum 2620 and local minimum 2622are representative of a correctly identified peak and valley of aphysiological pulse. Diastole period modulated PPG signal 2606, however,does not include any spurious local maxima and minima due to noise. Forexample, region 2624 of diastole period modulated PPG signal 2606 doesnot include a spurious local maximum and minimum whereas correspondingregion 2616 of systole period modulated PPG signal 2604 includes aspurious local maximum and minimum.

In view of the foregoing, for pulse identification techniques in thepresence of moderate noise, the diastole period cardiac cycle modulationtechnique may provide improved performance. For example, diastole periodcardiac modulation may result in reduced identification of spuriouspairs of incorrectly identified local maxima and minima.

Pulse amplitude and variations thereof are common calculations inphysiological monitors. The simulated waveforms of plot 2600 of FIG. 26may be used to calculate pulse amplitudes (i.e., the differences betweenmaxima and minima). The calculations may be computed on the maxima andminima of correctly identified pulses. Based on this analysis, it may bedetermined that noise contributes coefficients of variation of 2.6%,1.9%, and 3.8% to the computed pulse amplitudes of PPG signal 2602,systole period modulated PPG signal 2604, and diastole period modulatedPPG signal 2606, respectively. Accordingly, for pulse amplitudecalculation techniques in the presence of moderate noise, the systoleperiod cardiac cycle modulation technique may provide improvedperformance.

FIG. 27 is illustrative plot 2700 of waveforms showing dicrotic notchidentification in accordance with some embodiments of the presentdisclosure. Plot 2700 may include PPG signal 2702, notch high modulatedPPG signal 2704, and notch low modulated PPG signal 2706. “Notch high”will be understood to represent a cardiac cycle modulation techniquewhere the light source brightness is relatively high during the dicroticnotch. “Notch low” will be understood to represent a cardiac cyclemodulation technique where the light source brightness is relatively lowduring the dicrotic notch. For example, notch high modulated PPG signal2704 may include a signal where the phase relationship between pulse andLED output has been shifted so that the maximum LED output is alignedwith the occurrence of the dicrotic notch. In a further example, thenotch low modulated PPG signal 2706 may include a signal where the phaserelationship between pulse and LED output has been shifted so that theminimum LED output is aligned with the occurrence of the dicrotic notch.Phase relationships may be determined, for example, using informationfrom prior pulse cycles. Plot 2700 may be shown with arbitrary units onthe ordinate axis and time on the abscissa. The signals shown in plot2700 are simulated sawtooth-shaped 60 BPM IR waveforms with a moderateamount of noise (e.g., Gaussian noise from approximately 0-5 Hz withamplitude independent of emitter output). The signals provide examplesthat illustrate when cardiac cycle modulation is properly selected, theaccuracy of monitoring functions can be enhanced. The circles shown inplot 2700 indicate the occurrence of pulse minima and notch minima.Minima may be identified using any suitable processing technique. Insome pulse cycles, pulse and or notch minima may be obscured by noiseand not indicated.

PPG signal 2702 may be representative of a PPG signal determined using afixed power output throughout the cardiac cycle. Identified pulseminimum 2708 and dicrotic notch minimum 2710 are representative of acorrectly identified dicrotic notch in a physiological pulse in PPGsignal 2702.

Notch high modulated PPG signal 2704 may be representative of a PPGsignal determined using a cardiac cycle modulation, where the lightsource is modulated with a sinusoidal waveform that varies thebrightness of the emitter from, for example, 50-150% of the mean. Thepeak emitter output is centered during the dicrotic notch period of thecardiac cycle. For example, the light drive signal may drive an IRmodulated LED using sinusoidal cardiac cycle modulation where peak LEDoutput occurs during the dicrotic notch. In a further example, thecardiac cycle modulations may relate to cardiac cycle modulationsillustrated in plot 700 of FIG. 7 and plot 1200 of FIG. 12. Identifiedpulse minimum 2712 and dicrotic notch minimum 2714 are representative ofa correctly identified dicrotic notch in a physiological pulse in notchhigh modulated PPG signal 2704.

Notch low modulated PPG signal 2706 may be representative of a PPGsignal determined using a cardiac cycle modulation, where the lightsource is modulated with a sinusoidal waveform that varies thebrightness of the emitter from, for example, 50-150% of the mean. Theminimum emitter output is centered during the dicrotic notch of thecardiac cycle. For example, the light drive signal may drive an IRmodulated LED using notch low cardiac cycle modulation where minimum LEDoutput occurs during the dicrotic notch. In a further example, thecardiac cycle modulations may relate to cardiac cycle modulationsillustrated in plot 700 of FIG. 7 where the “on” period of the red lightdrive signal is indicative of the cardiac cycle minimum, and plot 1200of FIG. 12 where the “on” period of the light drive signal is indicativeof the cardiac cycle minimum. Identified pulse minimum 2716 and dicroticnotch minimum 2718 are representative of a correctly identified dicroticnotch in a physiological pulse in notch high modulated PPG signal 2704.Because notch low period modulated PPG signal 2706 uses lower powerduring the dicrotic notch portion of the cardiac cycle, noise may have agreater influence on the detection of the pulse minimum and dicroticnotch minimum of notch low modulated PPG signal 2706. In someembodiments, the increased effect of noise during the dicrotic notchportions may reduce the accuracy or reliability of a pulsedetermination.

Dicrotic notch and variations thereof are common calculations inphysiological monitors. Continuous non-invasive blood pressuremeasurements may be based on a differential pulse transit time. Thedifferential pulse transit time may be based in part on the interval,magnitude, or interval and magnitude of the dicrotic notch, relative tothe pulse minimum. The simulated waveforms of plot 2700 of FIG. 27 maybe used to calculate the notch interval and magnitude (i.e., thedifferences between the pulse minimum and the dicrotic notch minimum).The calculations may be computed on the pulse minimum and notch minimumof correctly identified cycles. Based on this analysis, it may bedetermined that the notch interval standard deviation was 24 ms, 14 ms,and 32 ms for the computed dicrotic notches of PPG signal 2702, notchhigh modulated PPG signal 2704, and notch low modulated PPG signal 2706,respectively. It may be determined that the notch magnitude standarddeviation was 4.0% of the pulse amplitude, 1.9% of the pulse amplitude,and 6.2% of the pulse amplitude for the computed dicrotic notches of PPGsignal 2702, notch high modulated PPG signal 2704, and notch lowmodulated PPG signal 2706, respectively. Accordingly, for dicrotic notchrelated calculation techniques in the presence of moderate noise, thenotch high cardiac cycle modulation technique may provide improvedperformance. In some embodiments, optimal cardiac cycle modulation foridentification or analysis of the dicrotic notch may include a maximumoutput centered on the dicrotic notch, which may be slightly after thesystole period. More generally, a modulation technique with a maximumemission centered on any pulse feature may increase the accuracy orreliability of that particular pulse feature.

FIG. 28 is illustrative plot 2800 of waveforms showing PPG signals inaccordance with some embodiments of the present disclosure. Plot 2800may include PPG signal 2802, systole period modulated PPG signal 2804,and diastole period modulated PPG signal 2806. Plot 2800 may be shownwith arbitrary units on the ordinate axis and time on the abscissa. Thesignals shown in plot 2800 are simulated sawtooth-shaped 60 BPM redwaveforms with a moderate amount of noise (e.g., Gaussian noise fromapproximately 0-5 Hz with amplitude independent of emitter output). Thered waveforms may be 25% of the intensity of the IR waveforms, as mayoccur in patients with dark skin pigmentation. As a result, the signalsillustrated in FIG. 28 may have reduced signal quality as compared to,for example, plot 2600 of FIG. 26. The signals provide examples thatillustrate that when cardiac cycle modulation is properly selected, theaccuracy of monitoring functions can be enhanced.

Systole period modulated PPG signal 2804 may be representative of a PPGsignal determined using a cardiac cycle modulation, where the lightsource is modulated with a sinusoidal waveform that varies thebrightness of the emitter from 50-150% of the mean. The peak emitteroutput is centered during the systole period of the cardiac cycle. Forexample, the light drive signal may drive a red modulated LED usingsinusoidal cardiac cycle modulation where peak LED output occurs duringsystole. In a further example, the cardiac cycle modulations may relateto cardiac cycle modulations illustrated in plot 1100 of FIG. 11.

Diastole period modulated PPG signal 2806 may be representative of a PPGsignal determined using a cardiac cycle modulation, where the lightsource is modulated with a sinusoidal waveform that varies thebrightness of the emitter from 50-150% of the mean. The peak emitteroutput is centered during the diastole period of the cardiac cycle. Forexample, the light drive signal may drive red modulated LEDs usingsinusoidal cardiac cycle modulation where peak LED output occurs duringdiastole. In a further example, the cardiac cycle modulations may relateto cardiac cycle modulations illustrated in plot 1000 of FIG. 10.

Ratio-of-ratio calculations are common calculations in pulse oximeters.Ratio-of-ratio calculations may use IR and red (or some othercombination of wavelengths) to determine oxygen saturation. Thecalculation may include wavelength-dependent and empirically determinedcalibration factors. The ratio-of-ratio calculation term may beapproximated as:

$\frac{\left( \frac{{Red}_{\max} - {Red}_{\min}}{{Red}_{mean}} \right)}{\left( \frac{{IR}_{\max} - {IR}_{\min}}{{IR}_{mean}} \right)}$where terms relate to the maximum, minimum, and mean amplitudes of theassociated signals. In some embodiments, the term may be calculated foreach pulse cycle.

The simulated waveforms of plot 2800 of FIG. 28 may be used to calculatethe ratio-of-ratios. The ratio-of-ratios term for the signals of plot2800, in the absence of noise may be 1.0 for each pulse cycle. Based onthe analysis of illustrated signals with included noise, it may bedetermined that the mean ratio-of-ratios term is 1.013 with a standarddeviation of 0.243, 0.995 with a standard deviation of 0.164, and 1.103with a standard deviation of 0.511 for the computed pulse cycles of PPGsignal 2802, systole period modulated PPG signal 2804, and diastoleperiod modulated PPG signal 2806, respectively. Accordingly, forratio-of-ratios calculation techniques in the presence of moderatenoise, the systole period cardiac cycle modulation technique may provideimproved performance.

The foregoing is merely illustrative of the principles of thisdisclosure and various modifications may be made by those skilled in theart without departing from the scope of this disclosure. The abovedescribed embodiments are presented for purposes of illustration and notof limitation. The present disclosure also can take many forms otherthan those explicitly described herein. Accordingly, it is emphasizedthat this disclosure is not limited to the explicitly disclosed methods,systems, and apparatuses, but is intended to include variations to andmodifications thereof, which are within the spirit of the followingclaims.

What is claimed:
 1. A pulse oximetry monitoring system, comprising: anoximetry sensor comprising a light source and a detector, wherein thelight source is configured to emit a light signal and wherein thedetector is configured to receive the light signal after attenuation bya subject, wherein the light signal comprises physiological pulses; anda pulse oximeter comprising a processor, wherein the pulse oximeter iscoupled to the oximetry sensor and is configured to: determine a cardiaccycle modulation based on the light signal, wherein at least one oflight intensity, light source firing rate, and duty cycle of the cardiaccycle modulation varies substantially synchronously with thephysiological pulses; and generate a light drive signal for activatingthe light source to emit the light signal according to the cardiac cyclemodulation.
 2. The system of claim 1, wherein the light intensity variessubstantially synchronously with the physiological pulses.
 3. The systemof claim 2, wherein the light intensity comprises a high value and a lowvalue, and wherein the light intensity is varied so that the high valuecoincides with at least one of a systole period, a diastole period, adicrotic notch, a peak, and a trough of the physiological pulses.
 4. Thesystem of claim 2, wherein the light intensity is varied according to aperiodic waveform, wherein the periodic waveform comprises a pluralityof light intensity peaks, and wherein the light intensity peaks areconfigured to coincide with at least one of a systole period, a diastoleperiod, a dicrotic notch, a peak, and a trough of the physiologicalpulses.
 5. The system of claim 1, wherein the light source firing ratevaries substantially synchronously with the physiological pulses.
 6. Thesystem of claim 5, wherein the light source firing rate comprises a highvalue and a low value, and wherein the light source firing rate isvaried so that the high value coincides with at least one of a systoleperiod, a diastole period, a dicrotic notch, a peak, and a trough of thephysiological pulses.
 7. The system of claim 1, wherein the pulseoximeter is further configured to sample the light signal at a samplingrate, wherein the sampling rate varies substantially synchronously withthe physiological pulses of the subject.
 8. The system of claim 1,wherein the duty cycle varies substantially synchronously with thephysiological pulses.
 9. The system of claim 8, wherein the duty cyclecomprises a high value corresponding to a longer duty cycle and a lowvalue corresponding to a shorter duty cycle, and wherein the duty cycleis varied so that the high value coincides with at least one of asystole period, a diastole period, a dicrotic notch, a peak, and atrough of the physiological pulses.
 10. The system of claim 8, whereinthe duty cycle is varied according to a periodic waveform, wherein theperiodic waveform comprises a plurality of duty cycle peaks, and whereinthe duty cycle peaks are configured to coincide with at least one of asystole period, a diastole period, a dicrotic notch, a peak, and atrough of the physiological pulses.
 11. The system of claim 1, whereinthe cardiac cycle modulation is correlated with periods of interest inthe cardiac cycle.
 12. The system of claim 1, wherein the physiologicalpulses of the subject are cardiac pulses.
 13. The system of claim 1,wherein the physiological pulses of the subject are respiratory pulses.14. The system of claim 1, wherein the light source comprises at leasttwo emitters and wherein the light drive signal is configured toactivate the at least two emitters.
 15. The system of claim 1, whereinthe light signal comprises a first wavelength of light and a secondwavelength of light, wherein the pulse oximeter is further configuredto: analyze the first wavelength of light of the light signal; andgenerate the light drive signal based on the analysis of the firstwavelength of light.
 16. The system of claim 15, wherein the pulseoximeter is further configured to analyze the first wavelength of lightof the light signal to determine information related to thephysiological pulses of the subject.
 17. The system of claim 1, whereinthe pulse oximeter is further configured to: receive an externaltrigger; and determine the cardiac cycle modulation based on theexternal trigger.
 18. The system of claim 1, wherein the pulse oximeteris further configured to determine a physiological parameter of thesubject based on the light signal.
 19. The system of claim 1, whereinthe at least one of light intensity, light source firing rate, and dutycycle of the cardiac cycle modulation is configured to vary during theperiod of each of the physiological pulses and substantiallysynchronously with at least one feature of the physiological pulses.